This replaces the somewhat hackish and usafe code used previously.
Also fixes a potential bug, where the newly created `local_id` was
dereferenced before checking for it to be non-null.
Seems to work OK in basic cases, but needs more work when copying
outside of Main at least.
Note: There is no behavioral changes expected from this commit.
Note that there are at least two known usecases for this change:
* Liboverrides, as with recursive resync and proxies conversion it
often ends up creating 'virtual' linked data that does not actually
exists in the library blend files.
* Complex versionning code (`do_versions_after_setup`) when it needs
to create new IDs (currently handling linked data that way is just not
supported!).
Implements #107847.
Bastien Montagne