Add support for installing flakes outside of nixpkgs

[seasonedfish]

A nix flake can be installed with e.g. nix profile install github:peterldowns/nix-search-cli.

hdn should support installing a flake with a similar syntax: hdn add github:peterldowns/nix-search-cli.

[Thesola10]

Most likely this could be achieved using

(builtins.getFlake "...").outputs.packages.${builtins.currentSystem}

plus minimal parsing of the #... part

[Thesola10]

Proposed state machine for flake URI parsing, which mostly matches native Nix behavior. NB: getFlake accepts the URI before the fragment verbatim.

There is no fragment in the flake URI

• flake.outputs.apps.${system}.default exists: use that
• flake.outputs.packages.${system}.default exists: use that
• flake.outputs.apps.${system} contains a single app: use it
• flake.outputs.packages.${system} contains a single app: use it
• fail

There is a fragment in the flake URI

• of the form #apps.a.b: use flake.outputs.apps.a.b
• of the form #packages.a.b: use flake.outputs.packages.a.b
• of the form #name (or anything else): if flake.outputs.name exists, use that, else try flake.outputs.apps.${system}.name then flake.outputs.packages.${system}.name

If the home-manager config is itself a flake, we may need to manage locking ourselves as getFlake only supports locked refs in pure evaluation mode (i.e. flake evaluation mode)

PS: don't forget to try legacyPackages after packages to retain compatibility with Nixpkgs and overlay flakes

[seasonedfish]

@Thesola10 thanks! I'll give that shot when I get the chance.