Fix WikiLinkPrefixer duplicating multi-part prefixes
The existing check in line #62 only works for 1-element prefixes. There
was no special case in place for multi-part prefixes. An existing link
[[w:de:…]] on the German Wikipedia that points to the German Wikipedia
would become [[:w:de:w:de:…]] during transfer. This should be a super-rare
edge-case, but was reported already.