mix-release: add ERL_LIBS paths to _build dir

pkgs/development/beam-modules/mix-release.nix

@@ -58,6 +58,20 @@ stdenv.mkDerivation (overridable // {
   configurePhase = attrs.configurePhase or ''
     runHook preConfigure
 
+    mkdir -p _build/$MIX_ENV/lib
+    while read -d ':' lib; do
+      for dir in $lib/*; do
+        # Strip version number for directory name if it exists, so naming of
+        # all libs matches what mix's expectation.
+        dest=$(basename $dir | cut -d '-' -f1)
+        echo "Linking $dir to _build/$MIX_ENV/lib/$dest"
+        # Symlink libs to _build so that mix can find them when compiling.
+        # This is what allows mix to compile the package without searching
+        # for dependencies over the network.
+        ln -s $dir _build/$MIX_ENV/lib/$dest
+      done
+    done <<< "$ERL_LIBS:"
+
     # this is needed for projects that have a specific compile step
     # the dependency needs to be compiled in order for the task
     # to be available