core: be more restrictive on the dependency types we allow to be created transiently

core: be more restrictive on the dependency types we allow to be created transiently

We should allow the ones that the [Unit] section of regular unit files
may accet, but no other, in particular not the internal deps we
synthesize as reverse of explicitly configured ones, such was WantedBy=.

Fixes: #14251
(cherry picked from commit 404308486aa285c67c5af4c8e7d6be393ce5a3c7)
(cherry picked from commit 26f3a534f1abba5284b1cc97eadb81ece960eb1a)