depmod: Stop opening modules.modinfo once per module

depmod: Stop opening modules.modinfo once per module

Since the addition of modules.aliases.bin, depmod has to open that
index multiple times and parse it over and over again:

	$ sudo strace -e openat  ./tools/depmod 2>&1 | grep modules.builtin.modinfo  | wc -l
	299
	$ time sudo  ./tools/depmod
	real    0m7.814s
	user    0m7.571s
	sys     0m0.237s

Rework the logic in depmod so it does everything: open, read and parse. The
format is very straightforward and we don't need to keep it in a data structure
since we only want to add the result to a index. New output:

	$ sudo strace -e openat  ./tools/depmod 2>&1 | grep modules.builtin.modinfo  | wc -l
	1
	$ time sudo  ./tools/depmod
	real    0m7.663s
	user    0m7.516s
	sys     0m0.139s

Indexes still match:

	$ cmp /tmp/modules.builtin.alias.bin.new
	/tmp/modules.builtin.alias.bin.old; echo $?
	0

Fix: https://github.com/kmod-project/kmod/issues/11