The ResourceLoaderSkinModule class after the introduction of https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/546733/ can be used by any skin module to achieve the same as adding the module mediawiki.skinning.interface
Just as we have deprecated mediawiki.skinning.content and mediawiki.skinning.elements we should deprecate this module too
= Acceptance criteria
[x] Vector instead of adding mediawiki.skinning.interface will now define their main class e.g. skins.vector.styles as a ResourceLoaderSkinModule
[x] Monobook instead of adding mediawiki.skinning.interface will now define their main modules as ResourceLoaderSkinModule
[x] mediawiki.legacy.shared will be added by Vector using the ResourceLoaderSkinModule feature `legacy`
[x] mediawiki.legacy.shared will be added by Monobook using the ResourceLoaderSkinModule feature `legacy`
[x] mediawiki.legacy.shared in core will be marked as deprecated
[x] CologneBlue should use a ResourceLoaderSkinModule
[x] All known impacted skins have been notified about the change to skins.
[x] Contents of mediawiki.legacy.oldshared will be moved to Nostalgia skin (https://gerrit.wikimedia.org/r/#/c/mediawiki/skins/Nostalgia/+/579413)
[x] Contents of mediawiki.legacy.oldshared will be moved to CologneBlue (https://gerrit.wikimedia.org/r/#/c/mediawiki/skins/CologneBlue/+/579415 )
[] mediawiki.legacy.oldshared in core will be marked as deprecated (https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/579416)
[] Styles in mediawiki.legacy.commonPrint should be moved to ResourceLoaderSkinModule (https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/579420) and marked as deprecated
= Developer notes
After more consideration I think `mediawiki.skinning.interface` in core should be retained for now for the FallbackSkin and API skin. We may in future want to rename this something like `skins.fallback.style` for consistency with other skins.