core: ProviderConfigTransformer properly locates parent module

Due to a logic error here we were trying to find our our module's parent
as a descendent of itself, rather than as a descendent of the root. It
turns out that we can do this even more simply by just accessing the
Parent field on the given config, avoiding the need to traverse the tree
down from the root at all.

While here, this also switches to using the path.Call helper method rather
than manually slicing the path array, since this better communicates our
intent.

terraform/transform_provider.go

@@ -510,12 +510,13 @@ func (t *ProviderConfigTransformer) addProxyProviders(g *Graph, c *configs.Confi
 		return nil
 	}
 
-	parentPath, callName := path[:len(path)-1], path[len(path)-1]
-	parent := c.Descendent(parentPath)
+	parentPath, callAddr := path.Call()
+	parent := c.Parent
 	if parent == nil {
 		return nil
 	}
 
+	callName := callAddr.Name
 	var parentCfg *configs.ModuleCall
 	for name, mod := range parent.Module.ModuleCalls {
 		if name == callName {