Handle several symlinks in system image generation

The symlinks below are added in fsProps:
system/product to /product
system/system_ext to /system_ext
system/vendor to /vendor
system/lib/modules to /system_dlkm/lib/modules

Test: CI and soong_generated_system_filesystem_test
Bug: 376490670
Change-Id: Ia1594e27593fce2e34e9601bc0516892fc33a3f7