Try to sort in rc-file order if not otherwise prevented

This MR would introduce a secondary build ordering to add to the ordering criteria, so that modules would build approximately in the order seen within the config file as defined by the user.

This permits the user some control in module ordering (as was previously the case), while still building per the dependencies specified in kde-build-metadata and prioritizing modules that are commonly depended upon to build earlier.

To support situations where kdesrc-build is used without an rc-file we still include the previous tiebreaker (sorting by module name) to ensure a reproducible build order for a given (and unchanged) set of dependency information and selectors.

See also #41 (closed), which inspired this MR.