Support base fields with multiple columns in entity queries

While not technically blocked on it, so not postponing, but it is touching the same code as #2068655: Entity fields do not support case sensitive queries, I'll start with this when that is in.

1. Hm.

taxonomy_term.description:
In 7.x, it was possible to query the format because the description value and format were two different properties. Now that we made it a single field with two properties, it is no longer possible.

that's not a very useful example (why would you do that..) but AFAIK, that is the only example that we have in core right now.

Just wanted to point this out, I don't care if this issue is critical or major, I plan to work on it asap anyway now that the other entity query issue is in.

Yes, I still want to fix this, major or critical. I was hoping that the entity query delta issue would have been done quickly, I was waiting on that one to avoid non-trivial conflicts in Tables.php, working with that class is hard enough without messy conflicts :)

Here's a test as a starting point. Will try to work on this soon.

Ok, I was hoping to use this issue to refactor \Drupal\Core\Entity\Query\Sql\Tables in this issue, instead of adding even more conditions, but it is probably better to go with a minimal fix here and test coverage so that we can get rid of this cri-major issue and refactor the code in a normal task.

Tables::addField() is now 150 lines of code with an insane amount of if/else/for with nasty $key++ in the middle of loops, and re-using of condition variable assignments 50 lines earlier. @Mile23 will probably suffer an instant heart attack if he will ever find this code. But as I said, that's something for another issue/day I think.

This passes EntityQueryTest and BreadcrumbTest. Go bot.

Comparing the "it is in a dedicated table" and "it is in a shared table" code branches now, looks like the only remaining difference feature-wise is the "follow relationship" thing (which TBH, confuses me a bit), that is still not supported for base fields. Do we want to open a separate issue for that ?

It confuses me more than a bit. The relationship code continues below, and it relies on this part. Because here we populate $propertyDefinitions there if it did not already happen here, *and* we run some additional code there. I tried to refactor this already when i changed it to support multi-value base fields (from config vs base field to separate table vs. shared table field). I tried to move all this code out of the if/else, but then the code below didn't work as expected anymore. So I duplicated it with different variable names to not change how that part there works. Fun, isn't it?

AFAIK, relationships for base fields actually work:

$ids = \Drupal::entityQuery('node')
  ->condition('uid.entity.name', 'admin')
  ->execute();

Works as expected. Don't ask me how or why.

So yes, we need a follow-up, but just for "Refactor \Drupal\Core\Entity\Query\Sql\Tables to avoid code duplication and improve readability" or something like that.

I think that's covered in https://github.com/drupal/drupal/blob/8.0.x/core/modules/system/src/Test...