Pass the *real* right path when building plugins.

Change-Id: Ib2019ad17874a4cc4239a10889cfed8b40f3a675
Reviewed-on: http://gerrit.dmdirc.com/2723
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Inject all the commands.

Instead of manually tracking dependencies and creating them in
CommandLoader, just have a module to provide a set of commands
that we can load.

This makes it stupidly easy to add dependencies in commands.

Also rename the /set command from Set because that's just
confusing.

Change-Id: I7967609e356de3c17d0942ebc913be438debe49c
Reviewed-on: http://gerrit.dmdirc.com/2722
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>