Fix bug when using --module foo and foo is a directory.

[NiltonVolpato]

• The current check would try to load that as a file and fail.
• Pointing to the file itself (foo/init.py) doesn't work because then the _is_submodule check fails, because the module name is a random UUID in that case.
• PYTHONPATH=. pydantic2ts.py --module foo --output foo.ts works in this case.