Fixes for tim.plunkett and dawehner review

The patch includes my fixes in #1952394-126: Add configuration translation user interface module in core, and fixes for @dawehner's comments.
The interdiff is just those new fixes.
I also fixed the bundle problem I mentioned, but it still needs tests.

It can't find stuff added in #2098089: Date formats cannot be translated (the core UIs are useless) :(

@tim.plunkett: first of all *huge* thanks for this review and for delivering it in a patch form. Amazing!

Second, I'm not sure what you mean in #3, but looks like #2098089: Date formats cannot be translated (the core UIs are useless) introduced the fails in this patch (but in a good way :D). It moved the language context to the global config contexts, so we need to just use it from the new place. (There may be other fails, but as with a module trying to follow how core changes, sometimes the code is just failing under us without this module changing a bit :D) I'm looking into fixing that fail right now.

Let's see how fixing the context will affect our fails :)

It helps if i actually include the change in the patch :D Same as #5 should have been.

Committed #7. We can either continue fixing / improving here to in separate issues. @timplunkett: we also have #2117711: Add tests to date format translation UI for test coverage for the newly added date type in core following #2098089: Date formats cannot be translated (the core UIs are useless).

Addressing #1952394-125: Add configuration translation user interface module in core points 2/3 by moving the entity discovery into the info hook instead of the plugin manager. This also let us remove 3 injected services from the plugin manager. :) This makes the manager work clearly on the meta level only while discovery happens with the .yml and info hooks only and not directly in the manager. Looking forward to what the testbot says :D

Now need to mock the module list and theme list stuff, since its in the constructor. Only the unit tests fail for that.

Meh.

Trying to mock list_themes() is pointless obviously since its a global function from theme.inc that is already defined :/ So we can disable looking at themes for now to continue with this patch. You'll notice this will pass because we are not actually testing that a theme could provide a .yml file. Yeah, needs to be fixed.

Now updated with fixes for the remaining points in #1952394-125: Add configuration translation user interface module in core. See also #1952394-132: Add configuration translation user interface module in core for detailed explanation on the createInstance() change.

Committed #15.

Opened #2123897: Restore theme .yml support, add/fix tests for theme .yml testing and restoring that stuff.