Autoloader: display an error message if a non-default vendor director…

…y is used (#15364)

The autoloader package only supports the default composer vendor directory. If
a consuming plugin uses a non-default vendor directory, generate an error during
composer dump-autoload.

packages/autoloader/README.md

@@ -42,3 +42,5 @@ Current Limitations
 -----
 
 We currently only support packages that autoload via psr-4 definition in their package.
+
+Your project must use the default composer vendor directory, `vendor`.

packages/autoloader/src/CustomAutoloaderPlugin.php

@@ -1,4 +1,4 @@
-<?php
+<?php //phpcs:ignore WordPress.Files.FileName.NotHyphenatedLowercase
 /**
  * Custom Autoloader Composer Plugin, hooks into composer events to generate the custom autoloader.
  *
@@ -71,11 +71,19 @@ public static function getSubscribedEvents() {
 	 */
 	public function postAutoloadDump( Event $event ) {
 
+		$config = $this->composer->getConfig();
+
+		if ( 'vendor' !== $config->raw()['config']['vendor-dir'] ) {
+			$this->io->writeError( "\n<error>An error occurred while generating the autoloader files:", true );
+			$this->io->writeError( 'The project\'s composer.json or composer environment set a non-default vendor directory.', true );
+			$this->io->writeError( 'The default composer vendor directory must be used.</error>', true );
+			exit();
+		}
+
 		$installationManager = $this->composer->getInstallationManager();
 		$repoManager         = $this->composer->getRepositoryManager();
 		$localRepo           = $repoManager->getLocalRepository();
 		$package             = $this->composer->getPackage();
-		$config              = $this->composer->getConfig();
 		$optimize            = true;
 		$suffix              = $config->get( 'autoloader-suffix' )
 			? $config->get( 'autoloader-suffix' )