Build and validate import metadata

[jdutant]

Citeproc crashes when the bibliography field is present but empty. It would be nice for collection to be able to validate import sources metadata before building them.

Currently the filter simply throws metadata at the imported file with --metadata-file flags, without e.g. first integrating the generic and child metadata specified in master. In a redesign it would first build the metadata first, validate it, and then send it to the imported file.