Using 'skin' as entrance point files, similar to already existing
convention in MediaWiki land with 'skin.json' or 'SkinVector.php' as
example in Vector skin. Replacing Apache inspired 'index' convention.
Also renaming legacy to 'skin-legacy' to be clear on file base that it's
a modification of 'skin'.
Bug: T249073
Change-Id: Ief1c469724d4ffe238d307407c3ddb46f2e1abfa
Volker E