fix last .IO.path.basename leftover
fix CompUnitRepo.files

When asking for an installed file (i.e. a script) then we basically ask for a filename, and
optionally for distname/auth/version. In contrast to asking for packages where we tend to ask
for packagename, and optionally for a filename/auth/version.
Configure: warning on missing --prefix option

requested for star in rakudo/star#40, and added here for consistency
Preserve warning message "0"
Merge branch 'nom' into newio