Add alias process service back (deprecated) and improve DX around BC path_alias services

First part, confirmed that this fixes the subpathauto tests, just need to install the entity type in kernel tests then and there are also existing, unrelated issues with the kernel test.

Here's a first attempt. Might not have gotten the deprecations quite right yet, but I found some interesting bits in PathAliasServiceProvider, for example it actually tries to deprecate path_processor_alias and path_subscriber but it fails to do so because these services actually don't exist anymore. So that was supposed to work but broken.

The problem with this idea of dynamically adding it is that it breaks static analysis including IDE's, so they don't know that path.alias_manager/storage are deprecated. so I think that should be there explicitly.

Also not sure why some services only had the deprecated comment above instead of the definition, the thing is that upgrade path tests are *allowed* to trigger deprecation messages, so as long as the fallback in CoreServiceProvider is only triggered in core, we're fine. seems to work OK with a few example tests that I did run locally, lets see about the rest.

The changes in core.services.yml might be a bit confusing, but I've attached a diff against core *before* the original patch was committed and it's pretty clear then, the only service that is removed is path.alias_repository because that was new in 8.8 and doesn't need BC. The others are there, with deprecated: definition just without tags. I've done the same in CoreServiceProvider btw, that means they wont' be called until the module is installed, but that's fine, we're not supposed to do caching and alias lookup during the upgrade path anyway.

I've also verified that together with the patch above, the patch from comment #5 in #3093401: Compatibility with Drupal 8.8.0-beta1 no longer shows a warning in pathauto.services.yml, although we can't commit that for different reasons.

Thanks for the reviews, some comments in case someone else wants to update it while I sleep (which I should be doing now)

#5.1: No can do, kernel tests now always install path_alias for BC: #3093257: Install path_alias by default in kernel tests to minimize the impact on contrib tests supporting 8.7.x, but I'd expect update tests to fail if things are missing, will do some testing, and we should also do some manual testing.

#5.2: yeah, that's always a bit ugly, actually explicitly removed it, due to the URL thing. But can add it back, no big deal. I think we leave it out on @deprecated on purpose.

#7: @see must only be used in a docblock and never as inline conmment. That can't be parsed in a meaningful way.

#7.1: I kind of like the pattern, but no strong feelings. if we wouldn't need the hasDefinition() check, maybe, but I think it that would make it quite repetitive, as each block would need to be in a if condition and have the service ID duplicated.

#7.2: Because unlike everything else, it's not a reference, it is actually a string.

Fairly sure we can still do it by overriding KernelTestBase::enableModules() :)

Nice idea, but setup doesn't use enableModules(), it does use \Drupal\KernelTests\KernelTestBase::getModulesToEnable() and \Drupal\KernelTests\KernelTestBase::getExtensionsForModules() to inject them directly into the kernel and these functions are private.

Maybe with some reflection magic, but I don't think this is worth it?

As I said, I did verify that the update test are failing if this isn't there, and they do, hard:

Symfony\Component\DependencyInjection\Exception\InvalidArgumentException : Unable to replace alias "path.alias_whitelist" with actual definition "path_alias.whitelist".
Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: You have requested a non-existent service "path_alias.whitelist".

In case you want to try yourself, this is the test I had:

namespace Drupal\KernelTests\Core\Path;

use Drupal\Core\EventSubscriber\PathSubscriber;
use Drupal\Core\Path\AliasManager;
use Drupal\Core\Path\AliasRepository;
use Drupal\Core\Path\AliasWhitelist;
use Drupal\Core\PathProcessor\PathProcessorAlias;
use Drupal\KernelTests\KernelTestBase;

/**
 * @group path
 * @group legacy
 */
class LegacyPathAliasServicesTest extends KernelTestBase {

  /**
   * Tests that the path_alias.module services have fallbacks defined.
   */
  public function testBCServices() {
    $this->assertFalse($this->container->get('module_handler')->moduleExists('path_alias'));
    $this->assertEquals(AliasRepository::class, get_class($this->container->get('path_alias.repository')));
    $this->assertEquals(AliasWhitelist::class, get_class($this->container->get('path_alias.whitelist')));
    $this->assertEquals(PathSubscriber::class, get_class($this->container->get('path_alias.subscriber')));
    $this->assertEquals(PathProcessorAlias::class, get_class($this->container->get('path_alias.path_processor')));
    $this->assertEquals(AliasManager::class, get_class($this->container->get('path_alias.manager')));
  }

minus non-working method overrides.

So, only fixed the two comment things, fine with adding dot, at least now for me in PHPStorm the . is not included in the URL now, not sure when it is and when not.

What about extending an existing method like this? kernel tests are fairly fast but still slow compared to unit tests, so I prefer to group things together when it's easy.

About the CR, IMHO we should update the existing one. I already think the two separate ones for the alias storage and the path_alias module are tricky and we should at least cross-reference them. I also had a look at modules using those services, the good thing is that there aren't too many:

http://grep.xnddx.ru/search?text=path_processor_alias
http://grep.xnddx.ru/search?text=path_subscriber