In MDLSITE-6096 it was agreed that se should be creating external functions under component\external[optional_subs]\. 1 external x class x file.

Of course, like with any coding style change, there isn't any impeding urgency of making the whole codebase to align with that agreement.

But it's ok to warn developers about the policy above, so this issue is about to add some Sniff able to:

• Find "incorrect" external uses: basically classes extending external_api not being in the expected location, maybe also by file name (externallib...).
• Warn about them pointing to the policy in the Docs and recommending to consider the move.

That's it.