reimport mesa 18, remove old MesaLib packages

[Tobias Nygren <tnn%NetBSD.org@localhost>]

Hello,

I am working on cleaning up the graphics/MesaLib18 package, updating
to the version 18.3.1 and validating on various $OPSYS.
After I am done I want to remove the unmaintained mesa version 7 and 11
from pkgsrc.

While doing this I wonder if now is a good time to rename the package
from graphics/MesaLib to just graphics/mesa or if we should stick with
the legacy package name?

Kind regards,
-Tobias