Gracefully omit the meta file from schema fragments

If the meta.json file is in the same directory as schema fragments,
it can currently get double-combined on `prmd combine`: once from the
--meta option and once from being in the given directory. As a
workaround users need to move meta.json out of the schema folder. This
change removes the need to use that workaround if users want the file
in the same directory.