Split out Soong's missing dependency option from Unbundled_build. There
are builds other than unbundled builds that don't have a full platform
tree (AOSP llvm branch is one example).
When this is set to true, Soong will defer the error for missing
dependencies until a module is attempted to be built, instead of when
it is reading and generating the build rules. So for platform builds it
will still fail early if something is missing.