better support for installing plugins via composer (#5112)

* merge installation and update commands
* generate metadata from array
* new command to list available packages
* added a management script to simplify updates
* added directory for dev files
* helper functions for installation and listing of packages
* run plugin database installers

tests/Command/InstallCommandTest.php

@@ -29,8 +29,7 @@ class InstallCommandTest extends KernelTestCase
         $container = self::$kernel->getContainer();
 
         $this->application->add(new InstallCommand(
-            $container->get('doctrine')->getConnection(),
-            $this->application->getKernel()->getEnvironment()
+            $container->get('doctrine')->getConnection()
         ));
     }