Legacy installables: ensure installable type is handled correctly

The legacy installables app needs to use `module` internally, but the
new installables app uses `extension`. We attempted to handle this in
2e516a7, but accidentally were modifying the `$slug` of the installable
instead. We also didn't correctly ensure that the installable objects
were still constructed with `module`, because that is what they expect.

See #404, 2e516a7

src/classes/installables.php

@@ -99,8 +99,8 @@ public static function register( $type, $slug, $data ) {
 		/** @var WordPoints_Installables_App $installables */
 		$installables = wordpoints_apps()->get_sub_app( 'installables' );
 		$installables->register(
-			$type
-			, 'module' === $slug ? 'extension' : $slug
+			'module' === $type ? 'extension' : $type
+			, $slug
 			, 'WordPoints_Installables::installer_loader'
 			, $data['version']
 			, ! empty( $data['network_wide'] )
@@ -263,6 +263,11 @@ public static function wpmu_new_blog( $blog_id ) {
 	 * @return WordPoints_InstallableI The installable object.
 	 */
 	public static function installer_loader( $type, $slug ) {
+
+		if ( 'extension' === $type ) {
+			$type = 'module';
+		}
+
 		return new WordPoints_Installable_Legacy(
 			$type
 			, $slug