fetchHex produces outputs that mix will accept

pkgs/development/beam-modules/fetch-hex.nix

@@ -19,9 +19,18 @@ stdenv.mkDerivation ({
   };
 
   unpackCmd = ''
-    tar -xf $curSrc contents.tar.gz
+    tar -xf $curSrc contents.tar.gz CHECKSUM metadata.config
     mkdir contents
     tar -C contents -xzf contents.tar.gz
+    mv metadata.config contents/hex_metadata.config
+
+    # To make the extracted hex tarballs appear legitimate to mix, we need to
+    # make sure they contain not just the contents of contents.tar.gz but also
+    # a .hex file with some lock metadata.
+    # We use an old version of .hex file per hex's mix_task_test.exs since it
+    # is just plain-text instead of an encoded format.
+    # See: https://github.com/hexpm/hex/blob/main/test/hex/mix_task_test.exs#L410
+    echo -n "${pkg},${version},$(cat CHECKSUM | tr '[:upper:]' '[:lower:]'),hexpm" > contents/.hex
   '';
 
   installPhase = ''