Configurable plugins can not control instance-specific config dependencies

So this is at least a beta target, maybe (probably) a beta blocker.

I had trouble understanding how this was different from e.g. menu blocks but this is the usecase:

I have a block plugin that doesn't use derivatives; instead, it adds to the block instance configuration. When I place an instance of this block, there's a form element where I can select the vocabularies that this block will display something-or-other for; could be one, or none, or several. So each block instance may have a dependency on one or more vocabularies. Since the config_dependencies key is part of the plugin definition as provided by derivatives -- not its configuration as stored in the block instance entity -- it doesn't help us.

Working on this.

This is bigger than blocks. I can reproduce in HEAD with actions and user roles:
For each user role, two actions are configured for adding and removing that role from a user.

The action export has no reference to these.

Here is an alternate solution, this moves all of the plugin related dependencies into one place. However, this needs an interface of some sort, and only works if you extend PluginBase.

I think #4 is much more clear in terms of who is responsible for what. Regular plugins do not care about dependencies, just configurable ones.
But #5 is cleaner code IMO, and makes the entity less aware of what plugins need.

I'm happy with either, and I defer to @Berdir/@alexpott.

Yeah this is beta-blocking then.

This is 4 rerolled. Ditching 5, it's too much for all plugins, and overkill.

Looks great. Minor clean up to use addDependencies() instead of addDependency() to avoid code repetition.

Hah, didn't know that was a method! That alleviates some of my worries about the ugliness of that, thanks.

+++ b/core/lib/Drupal/Component/Plugin/ConfigurablePluginInterface.php
@@ -36,4 +36,12 @@ public function setConfiguration(array $configuration);
+   * Calculates dependencies.

Whoa this is RTBC already. Awesome!

That said this needs MUCH better documentation...

Good point @xjm should have noticed that.

@alexpott and I came up with this expansion of the docs. We can add this to start, and then expand the scope of #2235363: Document config dependencies to configuration dependencies in general, because there's a lot more than that one key that needs documenting (as @tim.plunkett pointed out).

Sorry, one more fix.

rtbc if green

Looks great. Committed/pushed to 8.x, thanks!

This commit added an empty db.sqlite file to the repo. I suppose that should be deleted.

Missed that reviewing. Fixed locally and pushed.

This patch alters \Drupal\Component\Plugin\ConfigurablePluginInterface with references to \Drupal\Core, and effectively this is a core and not a component feature. Should we create \Drupal\Core\Plugin\ConfigurablePluginInterface and move this code there?

#25 I guess so yep

The references are to \Drupal\Core but the code doesn't introduce any dependency and could be used without core concepts.

I don't think we should move it, but not sure how to clean that up either.

Thanks!