AssetPlugin - Fail gracefully during uninstall

Before this patch, when uninstalling the plugin on composer v1, it would raise an error like:

```
Writing lock file
Generating autoload files
  - CiviCRM asset map
PHP Fatal error:  Uncaught Error: Class 'Civi\AssetPlugin\BasicAssetRule' not found in /home/me/src/sandbox/vendor/civicrm/civicrm-asset-plugin/src/Publisher.php:187
Stack trace:
0 /home/me/src/sandbox/vendor/civicrm/civicrm-asset-plugin/src/Publisher.php(215): Civi\AssetPlugin\Publisher->createAssetRule(Object(Composer\Package\CompletePackage))
1 /home/me/src/sandbox/vendor/civicrm/civicrm-asset-plugin/src/Publisher.php(98): Civi\AssetPlugin\Publisher->createAllAssetRules()
2 phar:///home/me/bknix/bin/composer/src/Composer/Plugin/PluginManager.php(196) : eval()'d code(125): Civi\AssetPlugin\Publisher->createAssetMap()
3 [internal function]: Civi\AssetPlugin\AssetPlugin_composer_tmp2->onAutoloadDump(Object(Composer\Script\Event))
4 phar:///home/me/bknix/bin/composer/src/Composer/EventDispatcher/EventDispatcher.php(164): call_user_func(Array, Object(Composer\Script\Event))
5 phar:///home/me/bknix/bin/composer/src/Composer/EventDispatcher/EventDispatcher.php(96): Composer\EventDispatcher\Event in /home/me/src/sandbox/vendor/civicrm/civicrm-asset-plugin/src/Publisher.php on line 187
```

src/AssetPlugin.php

@@ -118,7 +118,7 @@ class AssetPlugin implements PluginInterface, EventSubscriberInterface, Capable
    * @param \Composer\Script\Event $event
    */
   public function onAutoloadDump(Event $event) {
-    if ($this->checkConflicted()) {
+    if ($this->checkConflicted() || $this->checkRemoved()) {
       return;
     }
     $this->io->write("  - <info>CiviCRM asset map</info>");
@@ -131,7 +131,7 @@ class AssetPlugin implements PluginInterface, EventSubscriberInterface, Capable
   }
 
   public function runQueue(Event $event) {
-    if ($this->checkConflicted()) {
+    if ($this->checkConflicted() || $this->checkRemoved()) {
       return;
     }
 
@@ -193,4 +193,33 @@ class AssetPlugin implements PluginInterface, EventSubscriberInterface, Capable
     return $this->conflicted;
   }
 
+  /**
+   * In composer v1, the plugin class is loaded and activated before removal.
+   * This leaves us in a dangling state where uninstallation may raise errors.
+   *
+   * @return bool
+   *   TRUE if it appears that this has been removed.
+   */
+  protected function checkRemoved() {
+    try {
+      $pkg = $this->composer->getRepositoryManager()->getLocalRepository()->findPackage('civicrm/civicrm-asset-plugin', '*');
+    }
+    catch (\Exception $e) {
+      return TRUE;
+    }
+
+    if ($pkg === NULL) {
+      return TRUE;
+    }
+
+    try {
+      $path = $this->composer->getInstallationManager()->getInstallPath($pkg);
+    }
+    catch (\Exception $e) {
+      return TRUE;
+    }
+
+    return !is_readable($path);
+  }
+
 }