For historical reasons we still support naming of source files without
their extension.  Sources without known extensions are located on disk
by iterating through a fixed set of possible extensions.  We now want
users to always specify the extension, so the fixed set will not be
expanded and is preserved for compatibility with older projects.

This change adds recognition of extensions of all enabled languages to
avoid checking the disk for files whose extensions are unambiguous but
not in the original fixed set.