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

.gitignore

@@ -18,6 +18,7 @@
 # for keeping empty directories
 /config/packages/local.yaml
 /config/bundles-local.php
+/var/dev/*
 /var/data/*
 /var/cache/*
 /var/invoices*