releasetools: Switch from imp to importlib.util

imp is deprecated, and has been removed in python 3.12+.

This is largely following this example:

https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly

Bug: 388344853
Test: treehugger
Change-Id: I3b4f7659377951d7c1291f11a977516924b25393