Detect duplicates in rootmaps.(ROOT-6055).
Detect duplicates in rootmap files, but insert only if not in the map already. (ROOT-6055)
Correct the insertion of the rootmapfile entry in the already seen forward decls. (ROOT-6063 and 6141).
Change name of unoredered set of entries saw when parsing rootmap files. (ROOT 6063 and 6141).