Convert links in content to internal form?
In terms of non-technical (which I read as custom development) solutions, what you're describing is one of the features of LinkIt.


  Linkit provides an easy interface for internal and external linking
  with editors and fields by using an autocomplete field. Linkit links
  to nodes, users, managed files, terms and have basic support for all
  entities by default. Linkit has three major advantages over
  traditional linking


It provides a WYSIWYG toolbar button from which a user can paste a URL for embedding a link. Upon setting up a LinkIt profile for you input type, there's a Insert paths as setting with a Raw paths option that defaults the inserted link as Drupal's internal path.