Clean up iterate_over_symtabs

Clean up iterate_over_symtabs

After the "search via psyms" series, there's no need for
iterate_over_symtabs to first check the expanded symtabs -- the
callback will now be called for every relevant symtab, including ones
that were already expanded before the search.

Cleaning this up enables some other cleanups.  In particular,
iterate_over_some_symtabs is not needed in its current form, so here
I've renamed it, made it static, and made it search just a single
compunit symtab.

While there I cleaned up the "invert" logic in
objfile::map_symtabs_matching_filename.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30738
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>