A process that could not adopt a newly published dependency generation keeps
importing the old one. restart_needed() names that case (and stays silent for
dev venvs, Nix and anything not booted from a PM generation, so no false
restart prompts); adopt_selected() lets callers move onto the selection before
loading new code. The test helpers publish real generations and make the test
process run from one.
(cherry picked from commit 978abe8ec4b852778b8c63bcefdf141db51bc703)
(cherry picked from commit 28be27334adc531db59bf37a02a64acaaf47a2ee)
After a plugin's runtime request is published, the process that asked should be
able to import the result without a restart, but only when nothing it already
loaded would change underneath it. Adopt when this process was on the
generation that was selected when the build started, and no loaded module's
file appears in the RECORD of a distribution whose version changed or
disappeared. Adopting swaps the site-packages entry, processes only the new
generation's new .pth files, rewrites PYTHONPATH/PATH for child processes, and
leases the new generation while keeping the old lease, since loaded packages
still resolve submodules from it.
(cherry picked from commit 478f945d9f36e94229f58b251642b7c9f3b9e781)
