Backport Migrate 2.0 to Drupal 6

BTW, "we will now begin backporting" was expressed a little too optimistically - first we're doing some cleanup and refactoring of the D7 code (things like #753292: Factor out common bits of rollback/import), it'll be a lot easier to do it now in one branch than to do it later in two.

I've cut the DRUPAL-6--2 branch and populated it with straight copies of the D7 files - it's a long way from running at all, so please don't bother trying to check it out before we say it's ready for initial evaluation here.

FYI, the two primary issues remaining before backporting in earnest are:

#802264: Refactor map table support
#753284: Straighten out update scenarios

These involve significant refactoring, and thus would require a lot of extra work to backport individually after the initial backport is done.

Backport work has begun, and the basic infrastructure is ported, but there's still a ways to go around entities/fields. At the moment, focusing on simpletests to help guide the backport:

#392404: Develop simpletests for Migrate

I finally fixed comment import on D6 this past weekend, so Migrate 2 on D6 is in sync with D7. Still some work to go before both are ready for an alpha release, but I can close this issue out.