Add lookup_migrations configuration to menu_link_parent process plugin

In my opinion, the menu_link_parent process plugin should not be doing a migration lookup at all. That should be left to the migration plugin. Of course, this process plugin was added a long time ago, before the null_coalesce process plugin was added, so at the time it would have been awkward for the migration to handle it. For BC, we want to keep doing the same thing (by default).

Tests are passing. This issue is ready for review.

Nice improvement to an existing migration to make this more flexible. There's tests that are passing. LGTM.

I agree. This is fine. I updated credit.

One question on the MR - minor but not obvious to me what it should be (otherwise I'd have fixed on commit).

The confusing comment was a copy/paste artifact. I removed it.

Back to NR.

This can go back to RTBC.