content, directory loaders: Store validated nar as ExtIDs

After validating the nar integrity checksums provided by the lister, this now also
stores those as ExtID entries (as much as there are checksum hashes provided at
initialization time).

This behavior is shared by both Content and Directory loaders. This also only occurs
when the `checksums_computation` constructor parameter is set to "nar".

Refs. swh/meta#4979