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/xfce4-panel/template

@@ -1,6 +1,5 @@
 # Template file for 'xfce4-panel'
 pkgname=xfce4-panel
-sourcepkg=$pkgname
 version=4.6.1
 revision=1
 distfiles="http://mocha.xfce.org/archive/xfce-$version/src/$pkgname-$version.tar.bz2"