nixdb: Expression should contain correct hash

[jtojnar]

Problem to solve

When I want to use a Nix expression of a package, I need to manually replace nil hash with an actual one. It would be more convenient if the expression already had a correct hash.

User experience goal

The user should be just able to use the expression as is without any extra tweaking.

Proposal

We could store hash of each Nixpkgs revision in the database.