trivial-builders: add the applyPatches function

applyPatches applies a list of patches to a source directory.

For example to patch nixpkgs you can use:

  applyPatches {
    src = pkgs.path;
    patches = [
      (pkgs.fetchpatch {
        url = "1f770d2055.patch";
        sha256 = "1nlzx171y3r3jbk0qhvnl711kmdk57jlq4na8f8bs8wz2pbffymr";
      })
    ];
  }

pkgs/build-support/trivial-builders.nix

@@ -381,4 +381,37 @@ rec {
   # Copy a list of paths to the Nix store.
   copyPathsToStore = builtins.map copyPathToStore;
 
+  /* Applies a list of patches to a source directory.
+   *
+   * Examples:
+   *
+   * # Patching nixpkgs:
+   * applyPatches {
+   *   src = pkgs.path;
+   *   patches = [
+   *     (pkgs.fetchpatch {
+   *       url = "https://github.com/NixOS/nixpkgs/commit/1f770d20550a413e508e081ddc08464e9d08ba3d.patch";
+   *       sha256 = "1nlzx171y3r3jbk0qhvnl711kmdk57jlq4na8f8bs8wz2pbffymr";
+   *     })
+   *   ];
+   * }
+   */
+  applyPatches =
+    { src
+    , name ? (if builtins.typeOf src == "path"
+              then builtins.baseNameOf src
+              else
+                if builtins.isAttrs src && builtins.hasAttr "name" src
+                then src.name
+                else throw "applyPatches: please supply a `name` argument because a default name can only be computed when the `src` is a path or is an attribute set with a `name` attribute."
+             ) + "-patched"
+    , patches   ? []
+    , postPatch ? ""
+    }: stdenvNoCC.mkDerivation {
+      inherit name src patches postPatch;
+      preferLocalBuild = true;
+      allowSubstitutes = false;
+      phases = "unpackPhase patchPhase installPhase";
+      installPhase = "cp -R ./ $out";
+    };
 }