index flake inputs beyond nixpkgs

[KiaraGrouwstra]

nix-index now describes itself as a 'files database for nixpkgs'.
in practice, users may well end up using packages from more nix package repositories than those from nixpkgs.
as such, it would be nice if nix-index were generalized so as to say index packages among all of one's inputs, maybe treating nixpkgs as just one such input.

[KiaraGrouwstra]

implementing this seems more viable if one accepts flakes as a de facto part of today's standard, and probably includes:

• extending the database to include a field recording which flake a package is in
• the nix-env command
• its passed --file argument <nixpkgs>
• the command-not-found.sh script which should be generalized to not use a hardcoded toplevel=nixpkgs but actually name the correct flake as per the (extended) database
• .. maybe other bits using the database that would need updating as per such an extended schema