dotnetCorePackages.fetchNupkg: add override mechanism

pkgs/build-support/dotnet/fetch-nupkg/default.nix

@@ -6,6 +6,8 @@
   unzip,
   patchNupkgs,
   nugetPackageHook,
+  callPackage,
+  overrides ? callPackage ./overrides.nix { },
 }:
 {
   pname,
@@ -15,7 +17,8 @@
   url ? "https://www.nuget.org/api/v2/package/${pname}/${version}",
   installable ? false,
 }:
-stdenvNoCC.mkDerivation rec {
+let
+  package = stdenvNoCC.mkDerivation rec {
     inherit pname version;
 
     src = fetchurl {
@@ -72,4 +75,6 @@ stdenvNoCC.mkDerivation rec {
     '';
 
     createInstallableNugetSource = installable;
-}
+  };
+in
+overrides.${pname} or lib.id package

pkgs/build-support/dotnet/fetch-nupkg/overrides.nix

@@ -0,0 +1,9 @@
+{ autoPatchelfHook }:
+{
+  # e.g.
+  # "Package.Id" =
+  #   package:
+  #   package.overrideAttrs (old: {
+  #     buildInputs = old.buildInputs or [ ] ++ [ hello ];
+  #   });
+}