Remove/refactor internal/domains package

The following discussion from !541 (merged) should be addressed:

• @jaime started a discussion: (+1 comment)

  thought: I'd like to spend more time thinking about this package and whether or not we need a type Source interface anymore. There may be a way to just call internal/source/gitlab/. I'll get back to this tomorrow but if you have any ideas let me know. BTW this refactoring is not blocking this MR and can probably be done separately.