SI-7494 Each plugin must only be instantiated once.

The old behavior was that Plugin.loadAllFrom took a distinct
list of plugin locations.
            
The broken behavior incorrectly punted to the pruning
algorithm in Plugins.loadPlugins$pick.
                        
The new behavior is to distinctify the list of class names
to load.
                                    
This is more robust; under the old scheme, specifying the 
continuations.jar under two file names would fail as
described in the ticket.
                                                    
Pruning a plugin by disabling (-Xplugin-disable) or because
it steps on someone else's phase name suffers the same
defect. There is no way to tell the plugin it has been
benched. And you know that global I gave you?  Can I
have it back?

The second commit enables some pending tests, and I'll
supplement them in a future commit.