rust-matchers: better support file_set in IntersectionMatcher Previously, if both of the input matchers had non-exact file sets, IntersectionMatcher would simply have an empty fileset. Now, we duplicate Python behavior: we *union* the filesets of the matchers in that case. This makes some sense, because without exact input file sets, we can't do an exact intersection and must over-approximate. Concretely, this is necessary because the file_set is the list of files that were explicitly listed by the user, and we want to provide explicit errors for all such files, from both matchers.