Clear out loaded packages in PackageManager.prototype.reset

This way, when the AtomEnvironment is reset, packages will have .load() called
on them again, so their config schemas will be registered again.

src/package-manager.coffee

@@ -62,6 +62,7 @@ class PackageManager
   reset: ->
     @serviceHub.clear()
     @deactivatePackages()
+    @loadedPackages = {}
     @packageStates = {}
 
   ###