This fails when done via direct import into a cue slot which assumes that all sources created
must be associated with the region, and that each source must have a model.
This change drops the metadata-only track/source from being returned by Session::import_files()
Note that in order to avoid refcnt'ing from prolonging the life of the source we want to drop,
we do not announce the sources when they are created, but defer that until we know they are
going to be used post-import
1ee76f4178