Modify prebuilt_kernel_modules behave more like `build-image-kernel-modules-depmod`

This change defines a depmod staging directory for vendor_ramdisk
partition prebuilt kernel modules, and also modifies the build commands
of `modules.dep` to add the leading slash for vendor ramdisk kernel
modules.

Test: unpack and diff soong and make generated vendor ramdisk partitions
Bug: 381104942
Change-Id: I0d14ed676983976e20c7aca98b1c06ce097b1682