Move/Rename: goals/setup_py.py -> {util_rules,goals}/package_dists.py + subsystems/setup_py_generation.py

[cognifloyd]

In pants v1, there was a setup-py goal. In pants v2, that is now part of the more generic package goal. So, the name setup_py doesn't make sense any more.

Also, most of the rules in that file are used in contexts other than the package goal. For instance, I'm using them in #18639 in the export goal. And importing something from goals in util_rules feels rather odd. So, I would like to see most of these rules under util_rules.

This change does the following:

• renames: pants.backends.python.goals.setup_py to pants.backends.python.util_rules.package_dists
• adds a minimal: pants.backends.python.goals.package_dists
• moves SetupPyGeneration(Subsystem) to pants.backends.python.subsystems.setup_py_generation
• adjusts imports to use the new locations as needed.

[Eric-Arellano]

Good change! And thanks for keeping this focused on only moving files.