refactor(pypi): use a macro to define whl_library targets

[aignas]

Summary:

• refactor: Start implementing whl_library_targets
• refactor: start using whl_library_targets macro
• refactor: generate config settings in the new macro
• refactor: copy_files in the new macro
• refactor: move entry_point generation to the macro
• refactor: move the py_library and whl generation to the new macro

This makes the code more maintainable by reducing the amount of tests
that are comparing BUILD.bazel outputs.

[aignas]

FYI, I would like to include extra folders here, what me and @arrdem discussed in #1730. It would be valuable to have those files so that the hub repo can provide more filegroup targets out of the box. This may make things easier where the annotations API is heavily used by our users.

cc: @keith, not sure if this help in defining the cc_library targets, but regardless, this may be a good step forward.