Move module-specific migration support into the user module

Unblocked.

This guy is blocking #2534012: Move module-specific migration support into the file module.

WOW. That patch is like half of Migrate freakin' Drupal! Major kudos to @mikeryan for taking it on.

44.04 KB of very slow scrolling later, I think this looks pretty good. Git 'er done!

Needs a reroll.

It continues to please the eye and tickle the brain. (If by "tickle", you mean "smash with a sledgehammer".)

Needs reroll.

@mikeryan says this was just a re-roll, and no functionality changed. If it passes the tests, I'm satisfied.

@mikeryan and @phenaproxima walked me through this patch.

Even though this patch is quite large, the vast majority of this is just moving stuff around, so that's pretty straight-forward. A couple of weird things stuck out:

-migrate.destination.entity:user:
-  type: migrate_destination
-  label: 'User'
-  mapping:
-    md5_passwords:
-      type: boolean
-      label: 'Passwords'
-
 migrate.destination.entity:file:
   type: migrate_destination
   label: 'Picture'

Question: Why are we not moving the user picture at the same time we're moving the rest of the User stuff? Answer: Because that entry is mis-labeled. It has nothing to do with user pictures, but is rather about file entities, so will be addressed in #2534012: Move module-specific migration support into the file module.

+  /**
+   * {@inheritdoc}
+   */
+  public function getCountLog2($setting) {
+    return $this->originalPassword->getCountLog2($setting);
+  }
+

Uh... what the heck is this? :) Unlike other parts of this patch, this one has no corresponding - lines. Also, @inheritdoc is a misnomer; there's nothing in the parent class that defines this method; it actually happens in PhpassHashedPassword.

I asked for an updated comment here, and mikeryan posted:

/**
 * Implements the getCountLog2() method expected by tests assuming the PhpassHashedPassword service.
 * @todo: Revisit this whole alternate password service: https://www.drupal.org/node/2540594.
 */

I shortened this to:

  /**
   * Implements the PhpassHashedPassword::getCountLog2() method.
   *
   * @todo: Revisit this whole alternate password service:
   *   https://www.drupal.org/node/2540594.
   */

...and fixed on commit.

Committed and pushed to 8.0.x. Onwards with the great migration migration! ;)

Yes, yes.