occurrences: fix a bug with out-of-sync file detection when src files don't change sizes

[ikostia]

Summary

Prior to this change,find_linked_uids did not report out-of-sync files (unlike get_external_locs), despite observing them (log ~title "Found clashing idents %S <> %S. Ignoring UID %a."). This PR exposes this notion of a name clash (which may happen when a rename has just happened, but the file hasn't been rebuilt yet) in the return type of find_linked_uids: now the fn returns both the related UIDs and the files with clashes. The consumer fn locs_of just joins clashing_files with the out-of-sync files returned by get_external_locs.

Testing

r-stale-after-rename.t demonstrates how incompleteness-related log messages show up when something has been renamed by not rebuilt.