SourcePluginBase does not handle multi-value IDs

Problem/Motivation

Source plugins have the ability to filter on a list of IDs. Unfortunately, SourcePluginBase does the wrong thing when the IDs are multi-value (for instance, a field instance consists of an ID that has the node type and the field name) -- it just filters on the first value of the ID. The only reason this hasn't caused wacky behavior yet (or broken tests, for that matter) is that the ID list filtering functionality has not been used by Migrate Drupal, nor has it been specifically tested for.

Proposed Resolution

TBD. But whatever we decide, all PHPUnit tests of existing source plugins should add a test case which verifies that ID filtering works correctly.

Remaining Tasks

Figure out how to fix this, write the patch, review, and commit.

API Changes

None.

UI Changes

Zilch.