When installing FILE_SETs, it is possible the FILE_SET has not yet been
created with target_sources(FILE_SET). Instead of ignoring this
situation, we need to track the installed FILE_SET names and their
possible install destinations.
At generation time we resolve the names and destinations concretely. If
a FILE_SET wasn't provided or isn't an INTERFACE, we silently bail out.
Fixes: #26697
Vito Gamberini
d71b59a4f7