Make GetDirectDep implementation match comment

The comment on the interface of GetDirectDep states if there are
multiple deps with different tags, the first will be returned; however,
the current behavior is to panic if there are multiple deps. The
behavior now:
* a single dep, return the module and tag
* a single module with different tags: return module with first tag
* multiple modules: panic
* no module: return nil, nil

Bug: 186488405
Test: ~/aosp/build/bazel/ci/bp2build.sh
Test: m nothing
Change-Id: Id1e7315e7874b4a683ad7357ed2793822315821f