As preparation for composer plugins, rename custom installers to plugins
Implement a plugin manager and interface, update installer plugin tests
Plugin tests are no longer strictly installer tests
Load installed plugins at appropriate time and adapt tests accordingly
Turn EventDispatcher into generic solution handling plugins as well
Add an EventSubscriberInterface which may also be implemented by plugins
Fix typo
Plugins receive composer and io objects on construction already
Fix plugin interface usage in tests
Autoload dependencies of plugins using a pool of only the local repo
Add back --no-custom-installers option with a deprecated warning
Make composer/io part of the activate plugin API rather than constructor args
Correct authorship info for files I edited
Rename PrepareRemoteFilesystem event to PreFileDownload
Complete missing docblocks and fix incorrect ones
Fix disabling plugins which has to happen in the factory now
Revert constructor arguments to old order for custom installers
Use call_user_func for PHP < 5.4 compatability and accept __invoke
Load plugins from global vendor dir too
Load plugin code from global vendor dir correctly