[mypyc] Fix relative imports in __init__.pys

[Gobot1234]

Description

Fixes mypyc/mypyc#842

Test Plan

Re ran the actions and nothing failed, and it solves the issues with importlib.util.resolve_name failing.

Before:

(venv) James@Jamess-Air python-betterproto % mypyc src/betterproto/*.py

Traceback (most recent call last):
  File "/Users/James/PycharmProjects/python-betterproto/build/setup.py", line 5, in <module>
    ext_modules=mypycify(['src/betterproto/__init__.py', 'src/betterproto/_types.py', 'src/betterproto/_version.py', 'src/betterproto/casing.py'], opt_level="3"),
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/build.py", line 487, in mypycify
    groups, group_cfilenames = mypyc_build(
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/build.py", line 400, in mypyc_build
    group_cfiles, ops_text = generate_c(all_sources, options, groups, fscache,
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/build.py", line 200, in generate_c
    modules, ctext = emitmodule.compile_modules_to_c(
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/codegen/emitmodule.py", line 410, in compile_modules_to_c
    modules = compile_modules_to_ir(result, mapper, compiler_options, errors)
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/codegen/emitmodule.py", line 258, in compile_modules_to_ir
    scc_ir = compile_scc_to_ir(trees, result, mapper, compiler_options, errors)
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/codegen/emitmodule.py", line 209, in compile_scc_to_ir
    modules = build_ir(
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 79, in inner
    return func(*args, **kwds)
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/irbuild/main.py", line 80, in build_ir
    transform_mypy_file(builder, module)
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/irbuild/main.py", line 123, in transform_mypy_file
    builder.accept(node)
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/irbuild/builder.py", line 169, in accept
    node.accept(self.visitor)
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypy/nodes.py", line 379, in accept
    return visitor.visit_import_from(self)
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/irbuild/visitor.py", line 110, in visit_import_from
    transform_import_from(self.builder, node)
  File "/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/irbuild/statement.py", line 174, in transform_import_from
    id = importlib.util.resolve_name('.' * node.relative + node.id, module_package)
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/importlib/util.py", line 32, in resolve_name
    raise ImportError(f'no package specified for {repr(name)} '
src/betterproto/__init__.py:27: ImportError: no package specified for '._types' (required for relative module names)

After:

(venv) James@Jamess-Air python-betterproto % mypyc src/betterproto/*.py
running build_ext
building '999479a26e36c714bed7__mypyc' extension
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -arch x86_64 -g -I/Users/James/PycharmProjects/python-betterproto/venv/lib/python3.9/site-packages/mypyc/lib-rt -Ibuild -I/Users/James/PycharmProjects/python-betterproto/venv/include -I/Library/Frameworks/Python.framework/Versions/3.9/include/python3.9 -c build/__native_999479a26e36c714bed7.c -o build/temp.macosx-10.9-x86_64-3.9/build/__native_999479a26e36c714bed7.o -O3 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable
creating build/lib.macosx-10.9-x86_64-3.9
gcc -bundle -undefined dynamic_lookup -arch x86_64 -g build/temp.macosx-10.9-x86_64-3.9/build/__native_999479a26e36c714bed7.o -o build/lib.macosx-10.9-x86_64-3.9/999479a26e36c714bed7__mypyc.cpython-39-darwin.so
building 'betterproto.__init__' extension
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -arch x86_64 -g -I/Users/James/PycharmProjects/python-betterproto/venv/include -I/Library/Frameworks/Python.framework/Versions/3.9/include/python3.9 -c build/betterproto.c -o build/temp.macosx-10.9-x86_64-3.9/build/betterproto.o -O3 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable
creating build/lib.macosx-10.9-x86_64-3.9/betterproto
gcc -bundle -undefined dynamic_lookup -arch x86_64 -g build/temp.macosx-10.9-x86_64-3.9/build/betterproto.o -o build/lib.macosx-10.9-x86_64-3.9/betterproto/__init__.cpython-39-darwin.so
building 'betterproto._types' extension
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -arch x86_64 -g -I/Users/James/PycharmProjects/python-betterproto/venv/include -I/Library/Frameworks/Python.framework/Versions/3.9/include/python3.9 -c build/betterproto/_types.c -o build/temp.macosx-10.9-x86_64-3.9/build/betterproto/_types.o -O3 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable
gcc -bundle -undefined dynamic_lookup -arch x86_64 -g build/temp.macosx-10.9-x86_64-3.9/build/betterproto/_types.o -o build/lib.macosx-10.9-x86_64-3.9/betterproto/_types.cpython-39-darwin.so
building 'betterproto._version' extension
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -arch x86_64 -g -I/Users/James/PycharmProjects/python-betterproto/venv/include -I/Library/Frameworks/Python.framework/Versions/3.9/include/python3.9 -c build/betterproto/_version.c -o build/temp.macosx-10.9-x86_64-3.9/build/betterproto/_version.o -O3 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable
gcc -bundle -undefined dynamic_lookup -arch x86_64 -g build/temp.macosx-10.9-x86_64-3.9/build/betterproto/_version.o -o build/lib.macosx-10.9-x86_64-3.9/betterproto/_version.cpython-39-darwin.so
building 'betterproto.casing' extension
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -arch x86_64 -g -I/Users/James/PycharmProjects/python-betterproto/venv/include -I/Library/Frameworks/Python.framework/Versions/3.9/include/python3.9 -c build/betterproto/casing.c -o build/temp.macosx-10.9-x86_64-3.9/build/betterproto/casing.o -O3 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable
gcc -bundle -undefined dynamic_lookup -arch x86_64 -g build/temp.macosx-10.9-x86_64-3.9/build/betterproto/casing.o -o build/lib.macosx-10.9-x86_64-3.9/betterproto/casing.cpython-39-darwin.so
copying build/lib.macosx-10.9-x86_64-3.9/999479a26e36c714bed7__mypyc.cpython-39-darwin.so -> 
copying build/lib.macosx-10.9-x86_64-3.9/betterproto/__init__.cpython-39-darwin.so -> betterproto
copying build/lib.macosx-10.9-x86_64-3.9/betterproto/_types.cpython-39-darwin.so -> betterproto
copying build/lib.macosx-10.9-x86_64-3.9/betterproto/_version.cpython-39-darwin.so -> betterproto
copying build/lib.macosx-10.9-x86_64-3.9/betterproto/casing.cpython-39-darwin.so -> betterproto