Initial Nixpkgs mode

- To make this work, a lot of functions had to be moved to a different struct (IMO it's
  a strict improvement even on its own)
- I noticed that adding pins twice did not fail, so I added a check and a corresponding
  --force flag

README.md

@@ -217,6 +217,7 @@ USAGE:
 
 FLAGS:
     -n, --dry-run    Don't actually apply the changes
+    -f, --force      Overwrite existing pins with the same name
     -h, --help       Prints help information
 
 OPTIONS:

npins.nix

@@ -37,6 +37,7 @@ let
 
   cargoToml = builtins.fromTOML (builtins.readFile (src + "/Cargo.toml"));
   runtimePath = lib.makeBinPath [ nix nix-prefetch-git git ];
+
   self = rustPlatform.buildRustPackage {
     pname = cargoToml.package.name;
     version = cargoToml.package.version;
@@ -56,6 +57,10 @@ let
       wrapProgram $out/bin/npins --prefix PATH : "${runtimePath}"
     '';
 
+    postInstall = ''
+      ln -s $out/bin/npins $out/bin/npins-pkgs
+    '';
+
     meta.tests = pkgs.callPackage ./test.nix { npins = self; };
   };
 in