DirectoryLoader: Check nar hashes when provided

The lister now provides the "checksums_computation". This is either "standard" (for most
cases as in bare checksums on the object retrieved) or "nar" for some edge case. In that
case the computation is delegated to the "nix-store" command (which should be present in
the system running the loading).

This adapts the directory loader to deal with this case.

No work has been done for the ContentLoader yet besides failing the case if a call
happens with such case.

Related to T3781