shared: treat generator units as vendor units

Units created with "systemctl edit --full" from generator units aren't
removed with "systemctl revert" because units in the generator,
generator_early, and generator_late paths aren't considered vendor
units.

Alter path_is_vendor() to return true if the given path is a
generator unit, which in turn causes "systemctl revert" to remove units
created with "systemctl edit --full" as they now have vendor versions.

Fixes #10053