Add the symlinks of the external repository in api_bp2build workspace

Unlike bp2build, api_bp2build depends on a well-defined subset of checked-in
BUILD/bzl files, and we create symlinks for that subsset in
out/soong/api_bp2build. Since we now cd into the workspace after
aosp/2308141, we need to add the sylimks of the external repositories as
well

Test: bazel build --config=api_bp2build //:empty
Change-Id: Ia7ecc5543226893a3878bc0fd35e025a37c80413