Make sure prebuilt modules are exported before attempting to use them

Fixes an issue where prebuilt module under unexported soong
namespace overrides source modules when flag `prefer` was set.

Test: Create two modules under the same name, use cc_library_shared
      for first one and cc_prebuilt_library_shared with `prefer` set
      to true.
      Then put the prebuilt module under `soong_namespace {}` and
      attempt to compile it without appending the namespace path to
      product namespaces.
      Without this change it should fail to resolve the
      cc_library_shared module even if it should be available.

Change-Id: I506641218e6af94a0b5f048e09e06d83602e50f5