Add mediawiki-skin package type, modify mediawiki-extension package type

Name inflection for mediawiki-extension stays the same, except that a trailing
'-extension' is cut off if present.

Name inflection for mediawiki-skin will just cut off a trailing '-skin' if present.

That way package name collisions, e.g. between extension Bootstrap and skin
Bootstrap (they actually do exist both!), can be avoided.

The installation directory for skins is 'skins'.