Various bugfixes, including support for completing attribute paths when
supplying '' syntax as file input.

Motivation for this change

Some bug fixes and improvements:

• All commands should now complete attribute paths when supplying '<nixpkgs> as file input
• nix-env: -A only worked for -i/--install, now works for all main operations
• nix-env will now only complete main operations until one has been supplied
• nix-env --switch-generation will now complete generations
• nix-channel: completes channel names properly now

And a few other minor improvements.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.