libkmod: Prefer builtin index over builtin.alias

libkmod: Prefer builtin index over builtin.alias

The modules.builtin.alias.bin is way larger than the
modules.builtin.bin.  On a normal "distro kernel":

	21k modules.builtin.alias.bin
	11k modules.builtin.bin

From the kernel we get both modules.builtin and modules.builtin.modinfo.
depmod generates modules.builtin.bin and modules.builtin.alias.bin
from them respectively. modules.bultin is not going away: it's not
deprecated by the new index added. So, let's just stop duplicating the
information inside modules.builtin.alias.bin and just use the other
index.