feat(gazelle) Remove entry point file requirements when generating rules

[yushan26]

Remove entry point file requirements when generating rules. Enable python rule generation as long as there are .py source files under the directory so all new packages will have python rules generated in the package.

The extension used to require entrypoints for generation but:

• entry point for tests (i.e., __test__.py ) is no longer required after https://github.com/bazel-contrib/rules_python/pull/999 and https://github.com/bazel-contrib/rules_python/pull/2044
• entry point for binaries (i.e., __main__.py ) is no longer required after https://github.com/bazel-contrib/rules_python/pull/1584

The entry point for libraries (__init__.py ) shouldn't be required either, especially for Python 3.3 and after when namespace packages are supported.