Remove src/poetry/__init__.py to convert to namespace package (#5614)

* Remove `poetry.__init__` to convert to namespace package
* Add mypy settings to account for namespace package
* Update flake8 pre-commit config to allow namespace package
* Mark poetry-core as a third-party package in isort's config

.pre-commit-config.yaml

@@ -36,7 +36,6 @@ repos:
           - flake8-bugbear==22.4.25
           - flake8-comprehensions==3.10.0
           - flake8-eradicate==1.2.1
-          - flake8-no-pep420==2.3.0
           - flake8-quotes==3.3.1
           - flake8-simplify==0.19.2
           - flake8-tidy-imports==4.8.0

pyproject.toml

@@ -105,6 +105,7 @@ lines_between_types = 1
 lines_after_imports = 2
 src_paths = ["src", "tests"]
 extend_skip = ["setup.py"]
+known_third_party = ["poetry.core"]
 
 
 [tool.black]
@@ -117,6 +118,9 @@ force-exclude = '''
 
 [tool.mypy]
 files = "src"
+mypy_path = "src"
+namespace_packages = true
+explicit_package_bases = true
 show_error_codes = true
 strict = true
 enable_error_code = [