Shallow realisations should go back to using regular drv paths

[Ericson2314]

This works especially nicely in conjunction with #11896

For deep realisations, using "derivation hashes modulo" helps save space. But for shallow ones, it achieves nothing, since those are already keyed on resolved derivations, which are always distinct modulo ways of producing inputs.

For shallow ones, we should just go back to using store paths, which will be simpler, and save implementations that only care about CA derivations from having to implement the cumbersome and ATerm-encumbered "derivation hash modulo" algorithm.

This is tantamount to reverting bab1cda for shallow realisations; note that that affects all derivations.