Allow the idlist option to work on the primary ID of source plugins containing multiple IDs

This now has tests and is green -- is it ready for review?

I came upon this issue while exploring why --idlist was not working when I have two source id columns.

I am looking at the thread here, and the thread has tilted away from the original title (allow idlist to work with multiple source ids) to more of a "filter ids by pattern" type of request. While I am 100% in favor being able to specify ids based on a pattern (sounds very useful to me), it sounds like it would be better solved for by using a new migrate option, such as `--idfilter=` instead of trying to buckle this onto an option that is meant to be a very direct and simple list of source ids.

For filtering ids by pattern, I think we need a feature request (and a similar patch).

But this ticket and thread is about allowing migrations to work from the command line using --idlist as is, when you have more than one source column and I feel that issue is its own issue without mixing in filtering. Does this make sense?