Pytype cannot handle <class name>.<nested class name> in pyi files

[rchen152]

For example, third_party/2and3/google/protobuf/descriptor_pb2.pyi contains:

class UninterpretedOption(Message):
    class NamePart(Message):
        def FromString(cls, s: bytes) -> UninterpretedOption.NamePart: ...

Pytype thinks that UninterpretedOption is an import and goes looking for the corresponding pyi file, leading to:
third_party/2and3/google/protobuf/descriptor_pb2.pyi: pytype.load_pytd.BadDependencyError: Can't find pyi for 'google.protobuf.descriptor_pb2.UninterpretedOption', referenced from 'google.protobuf.descriptor_pb2'

The problem is that the CollectDependencies visitor assumes that the first part of a dotted name is always a module: https://github.com/google/pytype/blob/master/pytype/pytd/visitors.py#L1830.

[rchen152]

One thing that would be useful until we get this bug fixed is to give a clearer error message, so that when it is encountered in hand-crafted stubs, the writer can figure out what's going on.