makepkg: Add fallback to package function
makepkg looks for a package() function when building a single package
but package_$pkgname() style package functions when building a split
package. This patch allows the use of a package_$pkgname() function
when building a single package for consistency. This is achieved by
having makepkg consider a non-split package with a package_$pkgname()
function as a split package (creating just the one package).
Fixes FS#16622.
Signed-off-by: Allan McRae <allan@archlinux.org>
Signed-off-by: Dan McGee <dan@archlinux.org>