mkNugetSource: Also copy .nupkg files from subdirectories

Previously only .nupkg files directly in the deps directory were copied.
This is a regression because it breaks `projectReferences = [ ... ];` in
buildDotnetModule.

pkgs/build-support/dotnet/make-nuget-source/default.nix

@@ -15,12 +15,10 @@ let
     buildCommand = ''
       mkdir -p $out/{lib,share}
 
-      (
-        shopt -s nullglob
-        for nupkg in ${lib.concatMapStringsSep " " (dep: "\"${dep}\"/*.nupkg") deps}; do
-          cp --no-clobber "$nupkg" $out/lib
-        done
-      )
+      # use -L to follow symbolic links. When `projectReferences` is used in
+      # buildDotnetModule, one of the deps will be a symlink farm.
+      find -L ${lib.concatStringsSep " " deps} -type f -name '*.nupkg' -exec \
+        cp --no-clobber '{}' $out/lib ';'
 
       # Generates a list of all licenses' spdx ids, if available.
       # Note that this currently ignores any license provided in plain text (e.g. "LICENSE.txt")