Allow source spec to be a file

Author: angerman

compiler/ghc/configured-src.nix

@@ -19,7 +19,7 @@ stdenv.mkDerivation (rec {
     patches = ghc-patches;
     name = "${targetPrefix}ghc-${ghc-version}-configured-src";
 
-    # Make sure we never relax`$PATH` and hooks support for compatibility.
+    # Make sure we never relax`$PATH` and hooks support for compatability.
     strictDeps = true;
 
     nativeBuildInputs = [
@@ -40,7 +40,9 @@ stdenv.mkDerivation (rec {
 
     postPatch = "patchShebangs .";
 
-    src = fetchurl { inherit (src-spec) url sha256; };
+    src = if src-spec ? file
+        then src-spec.file
+        else fetchurl { inherit (src-spec) url sha256; };
 
     # GHC is a bit confused on its cross terminology.
     preConfigure = ''