RFC: Make nix-store packages relocatable

*WIP: I've created this ticket so I don't forget to flesh it out with a few possible approaches*

The basic idea/problem: we could easily have compiled nix-store packages made relocatable (and they're nicely self-contained in "$prefix/store/$compiler/$libname"), if `Paths_$pkgname.hs` would allow us to have a single variable for overriding the "$prefix/store" base-folder (strawman: "$CABAL_STORE_PATH"), rather than hardwirding the `$prefix` into each compiled artifact (to keep this simple, I'm going to ignore DSO-related path-relocability-issues).

TODO: describe minimal/specific variant, as well as a more general variant involving general var-name template interpolation scheme.

/cc @ezyang @dcoutts