Switch plugin packages back to git submodules (#678)

Alright, so I hate to be the one to do this, but #584 broke crates.io
publishing and also caused librojo to be unusable. I see that there was
some discussion on Discord shortly after the problem was realized, but
there was no action taken.

I think keeping librojo and publishing working far, far outweigh any
convenience added by Wally.

I've kept the same `Packages` naming convention to keep the diff
minimal.

plugin/test-place.project.json

@@ -9,7 +9,7 @@
       },
 
       "Packages": {
-        "$path": "DevPackages"
+        "$path": "Packages"
       }
     },