Simplify plugin loader code.

* use one place to build plugin file names,
 * use one loader instance,
 * test for plugin enabled flag in just one place to
   avoid errors and reduce if statements,
 * don't return private objects,
 * factor goplugin loading to a method,
 * fix a related test that was commented out.

pkg/plugins/compiler.go

@@ -62,8 +62,7 @@ func DefaultSrcRoot() (string, error) {
 	}
 	nope = append(nope, root)
 
-	root = filepath.Join(
-		pgmconfig.ConfigRoot(), plugin.PluginRoot)
+	root = plugin.DefaultPluginConfig().DirectoryPath
 	if FileExists(root) {
 		return root, nil
 	}