REST API: Fix fatal error on multisite when calling the single item plugin routes.

The `is_network_only_plugin` and `is_plugin_active` functions are not available in a front-end context and must be specifically loaded.

This commit brings the changes from [49952] to the 5.6 branch.

Props oakesjosh, johnbillion, TimothyBlynJacobs.

Fixes #52205.

Built from https://develop.svn.wordpress.org/branches/5.6@50046


git-svn-id: http://core.svn.wordpress.org/branches/5.6@49747 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api/endpoints/class-wp-rest-plugins-controller.php

@@ -209,6 +209,8 @@ class WP_REST_Plugins_Controller extends WP_REST_Controller {
 	 * @return true|WP_Error True if can read, a WP_Error instance otherwise.
 	 */
 	protected function check_read_permission( $plugin ) {
+		require_once ABSPATH . 'wp-admin/includes/plugin.php';
+
 		if ( ! $this->is_plugin_installed( $plugin ) ) {
 			return new WP_Error( 'rest_plugin_not_found', __( 'Plugin not found.' ), array( 'status' => 404 ) );
 		}

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6.1-alpha-50045';
+$wp_version = '5.6.1-alpha-50046';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.