This rovides an abstraction to fetch a worldedit command's definition,
regardless of where it was registered.
We would normally expect all commands to be registered in
wea_c.registered_commands, but before we only do a one-off pass to
import commands from worldedit should a new mod we aren't aware of
register a command with worldedit and get loaded after us, we won't
detect it - hencee the need for this function.
