Make DirectoryWithMetadataDiscovery generic and reusable

1. What contrib modules are waiting for this?
2. In Drupal core, we generally only introduce an abstraction once there's >=3 uses for it.

What contrib modules are waiting for this?

sdc_styleguide → and UI Patterns 2.x's ui_patterns_library sub-module are working on a common stories format and we planned to use Drupal\Core\Plugin\Discovery\DirectoryWithMetadataDiscovery to load them.

I can see other modules, contrib or custom, which would want to load a plugins from {drupal_extension}/predefined/path{plugin_id}/{plugin_id}.{plugin_type}.yml

Anyway, even if it is not useful for anybody, we have this weird situation to address where 2 discovery classes which are not looking related to SDC, not documented as part of SDC, and located in the Drupal\Core\Plugin namespace, have a string (a single string!) tied to SDC:

    // Detect "my_component.component.yml".
    $regex = '/^([a-z0-9_-])+\.component\.yml$/i';

That makes sense! :) Thanks for explaining! I think that'll convince core committers too :)