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/gcc/template

@@ -1,6 +1,5 @@
 # Template build file for 'gcc'
 pkgname=gcc
-sourcepkg=$pkgname
 version=4.4.1
 patch_files="$pkgname-fix-amd64.diff"
 distfiles="