Unable to register collections from mu-plugins / theme directories

[spacedmonkey]

The register_collection method was added in [59132]. It allows block collections to be register from the incluces path and the plugins directory.

However, it should be possible to register a collection from the following directory.

mu-plugins
theme directory
child theme directory.

Registering from another these directories results in the following doing it wrong message.

  Function WP_Block_Metadata_Registry::register_collection was called incorrectly. Block metadata collections can only be registered for a specific plugin. The provided path is neither a core path nor a valid plugin path. (This message was added in version 6.7.0.)

[spacedmonkey]

CC @flixos90 who worked on the original patch.

[flixos90]

Duplicate of #62140.