Does not happen very often, but that weak handling of copying linked
data as linked data currently can lead to an invalid namemap in Main.
This is a known issue, fixing it requires addressing #107847.
In the mean time, work around it by re-validating and fixing the namemap
after the problematic liboverride calls.
NOTE: only identified issue currently is the proxy conversion of linked
proxies. The other cases *should* be fine.
Found while investigating issues when opening the
`lib/tests/libraries_and_linking/libraries/main_scene.blend` file.
ff126ede17