Set sourcepkg to pkgname through run_template().

This removes the need for having to specify sourcepkg manually in
every package that will build subpkgs.

--HG--
extra : convert_revision : 383e4c467e84844d2f4f61ae5c5e07a5124e9cfb

templates/webkit-gtk/template

@@ -1,6 +1,5 @@
 # Template file for 'webkit-gtk'
 pkgname=webkit-gtk
-sourcepkg=$pkgname
 version=1.1.8
 wrksrc=webkit-${version}
 distfiles="http://www.webkitgtk.org/webkit-${version}.tar.gz"