Add support for multi-output derivations

[jlesquembre]

Fixes #595

[avdv]

Instead of having to provide different flake package outputs in the flake.nix file, I would prefer introducing an optional output attribute:

rules_nixpkgs.flake_package(
    name = "curl-bin",
    nix_flake_file = "//:flake.nix",
    nix_flake_lock_file = "//:flake.lock",
    package = "curl",
    output = "bin",
)

This should result in a call to nix build --output-link "out" '<flake>#<package>^<output>'. This should obviate the need to parse the output of the command; we can directly infer the output link from the name.

[jlesquembre]

Good point, I've updated the PR to match that interface.