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 (closed)