Provide a data directory to plugins on demand

If a plugin requests a data directory with a @PluginData File
dependency, a data directory will be created automatically by the
server in $site_path/data/$plugin_name and passed to the plugin.

Plugins can use this to store any data they want.

Change-Id: I98de44a033889740cafc8a3d72c582c0e99b7ffd
5 files changed