A bunch of fixes, and now completes attribute paths when <nixpkgs>
syntax as file input is used.

Hopefully shouldn't be that many bugs left now :)

Motivation for this change

0.4:

• Fix nix-build -A nixUn completion.
• Fix nix-build some/path -A, the script didn't take realpath of some/path

0.3:

• 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

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.