Apply platform patches before planning source repository packages

scripts/nix/source-repository-patches.nix

@@ -0,0 +1,24 @@
+{ pkgs, cabalProject, sha256map, patchesByUrl }:
+let
+  parsed = pkgs.haskell-nix.haskellLib.parseSourceRepositoryPackages
+    "cabal.project" sha256map {} cabalProject;
+  repos = map (block: block.sourceRepo)
+    (builtins.filter (block: block ? sourceRepo) parsed.sourceRepos);
+  patchedSource = url: patches:
+    let
+      matches = builtins.filter (repo: repo.url == url) repos;
+      repo = assert pkgs.lib.assertMsg (builtins.length matches == 1)
+        "Expected exactly one pinned source repository for ${url}";
+        builtins.head matches;
+      rev = repo.rev or repo.ref;
+    in {
+      name = "${url}/${rev}";
+      value = pkgs.applyPatches {
+        src = pkgs.fetchgit {
+          inherit (repo) url sha256;
+          inherit rev;
+        };
+        inherit patches;
+      };
+    };
+in builtins.listToAttrs (pkgs.lib.mapAttrsToList patchedSource patchesByUrl)