Select stub/impl per apex variant

Create a select statement for every api_domain a library could be
included in. The stub/impl
selection heuristics per apex domain are
1. If dep has stubs and the api domain appears in dep's apex_available, use
   impl
2. If dep has stubs and the api domain does not appear in dep's
   apex_available, use stubs

(Category 3: If dep does not have stubs and the apex does not appear in
dep's apex_available, then a separate apex_available validation in Bazel
will emit an error).

Platform variants have been special-cased for now to use equality of
apex_available for stub/impl selection

Test: go test ./bp2build
Bug: 272378496

Change-Id: Ibd29efd763c8863c7e6d2a9af0da30bbde07175d