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 generate py_test without __test__ #999 and feat!: Following generation mode when generating test targets #2044
• entry point for binaries (i.e., __main__.py ) is no longer required after feat: Creating one py_binary per main module #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.

[dougthor42]

Dang, now I'll have to use some other method to enforce people to have __init__.py files 🤣

But yeah I'm fine with this. I'm curious as to why the generation_mode project directives were added though.

[dougthor42]

Oh, and please add a note to CHANGELOG.md, as some people (like me!) might rely on the fact that a BUILD.bazel wasn't generated if an __init__.py file wasn't present.

[yushan26]

This is an existing test that is testing if subdirectory srcs gets added to a py_library target. Subdirectory sources will only be added to a py_library target in project mode.