Generate cross partition kernel module dependencies

Kernel modules in vendor_dlkm might have dependency on system_dlkm. To
correctly generate modules.dep files, copy system_dlkm modules to depmod
staging dir before calling depmod.

Test: m, check $OUT/vendor_dlkm/lib/modules/module.dep, make sure it
contains dependency on /system/lib/modules
Bug: 149866755

Change-Id: Ibdf6d63df1413543471a3d18108eba6fdcde9ba4