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)
5d39ddd28b