[Cooker] plib is needed by tuxkart-0.0.3-3mdk --> plib not in cooker

[Stefan van der Eijk]

Plib seems to be required to build tuxkart, but plib is not in cooker
but in contrib. Plib is not required to install tuxkart.

Are there any guidelines on which packages have to be in cooker? Is it
that all packages required to run stuff in cooker should be in cooker
too, but stuff needed to build cooker can be outside cooker?

Stefan