Rename pluginjsonmetadata.xsl -> qtcreatorplugin2json.xsl

The original name inconveniently starts with 'plugin', creating a
conflict for shell tab completion when trying to enter src/plugins/...
taking extra keystrokes to resolve.

Change-Id: I7fa59d767aa900f73cd643dfab0d5b2131652a2d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>