Move/restore four migrate process plugins to the migrate module

Benji Fisherrequested to merge
issue/drupal-3560075:3560075-move-process-plugins-main into main

The comments should not assume that the code is being used in the context of a site migration.

Testing shows that parse_str($a['query'], $a['query']) works the way we want it to.

Closes #3560075

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