Make BuildDependencies construction a proper function outside of the settings sytem.
'plugins' and 'plugin <name>' commands to inspect plugins

* 'plugins' displays the list of plugins available for each build along with the project IDs each is enabled on
* 'plugin <name>' displays information about a specific plugin in the context of the current project
 - if the plugin is activated on the current project and if so, information about the keys/configurations it provides
 - how the plugin could be activated if possible
* tries to detect when it is run on an aggregating project and adjusts accordingly
 - indicates if an aggregated project has the plugin activated
 - indicates to change to the specific project to get the right context

This is a rough implementation and needs lots of polishing and deduplicating.
The help for the commands needs to be added/expanded.