New subprojects packagefiles subcommand

It is now possible to re-apply meson.build overlays (patch_filename or patch_directory in the wrap ini file) after a subproject was downloaded and set up, via meson subprojects packagefiles --apply <wrap-name>.

It is also possible for patch_directory overlays in a [wrap-file], to copy the packagefiles out of the subproject and back into packagefiles/<patch_directory>/ via meson subprojects packagefiles --save <wrap-name>. This is useful for testing an edit in the subproject and then saving it back to the overlay which is checked into git.