Changes to enable executing named Seed from within the seedFiles found on the path.

Tests included.
This is useful for my integrationTests where I would like to share datasource state between test classes without needing to install all seed data. 
Handles seed references by SeedName and PluginName.SeedName in same way as the dependency resolver.  However, I found I needed to convert from PluginName to plugin-name in order to find the correct reference.