feat(package): use explicit fileset for package src

This uses a fileset to restrict the amount of rebuilds that one has
to perform.

For example, if a test is changed, we do not want to rebuild the
`nixos-cli` package, since that's wasteful. Using an explicit opt-in
fileset for src works here because there are only a few files/directories
required to compile the actual package, and they can be specified
manually without much trouble.

Author: water-sucks

nix/package.nix

@@ -1,7 +1,6 @@
 {
   lib,
   buildGoModule,
-  nix-gitignore,
   installShellFiles,
   stdenv,
   scdoc,
@@ -11,7 +10,19 @@
 buildGoModule (finalAttrs: {
   pname = "nixos-cli";
   version = "0.15.0-dev";
-  src = nix-gitignore.gitignoreSource [] ../.;
+
+  src = lib.fileset.toSource {
+    root = ../.;
+    fileset = lib.fileset.unions [
+      ../go.mod
+      ../go.sum
+      ../Makefile
+      ../main.go
+      ../cmd
+      ../doc
+      ../internal
+    ];
+  };
 
   vendorHash = "sha256-I04cLEXLTRbxxCBemj4J6JzrtDaUQS0TuZZ8Fa0EZBk=";