devel/py-pygobject: rename and update to 3.50.0

[vishwin]

Summary

The proper upstream Python package name is pygobject; version 2 hasn't existed or been supported in ages, so rename.

Rework the port to where -common is the child port but the main port depends on it for the header files. Build the main port as a Python wheel USE_PYTHON=pep517 to take advantage of USE_PYTHON=autoplist.

Test Plan

dogfooded for some time already, exp-run

[vishwin]

rebase

• devel/pygobject-common: shift consumers
• security/libsecret: shift TEST_DEPENDS

USE_GNOME=pygobject3 consumers to be bumped post-commit