Favor importlib.metadata.packages_distributions() over
importlib_metadata.packages_distributions() where it is available, i.e.,
on Python 3.10 and later.
Depend on PyPI importlib_metadata only on Python 3.9 and older:
conditionalize the requirements.txt entry on Python version.
Change-Id: I624fc0059057de05bb2bacd9c1238cba71ebad2a