I think some (unofficial) place already offers a finished index
channels come with index of binaries, so e.g.
$ rustc
The program 'rustc' is not in your PATH. It is provided by several packages.
You can make it available in an ephemeral shell by typing one of the following:
nix-shell -p mrustc-bootstrap
nix-shell -p rustc
nix-shell -p rustup
But unfortunately, programs.sqlite maps executable name to package name, I need full file index. Maybe it will be useful for someone, programs.sqlite contains only in nixos channels. FYI you can download it from Channels for NixOS project(s)nixexprs.tar.xz file.