The use case for this is creating config_setting(s) specific to an apex
variant and selecting stub/impl in that config_setting. We likely need
only a handful of such config_setting(s), but determining that list
requires iterating the build graph.
Test: go test ./bp2build
Change-Id: I9aa552e3d0bcf67513023c3a7d4bbf8fae464ee4
We don't want rules to load the product variables directly, because
then they won't be able to transition on them.
Break constant information that is safe to load outside of the
product vars file, so that we can make the product vars file have
more restricted visibility later.
Bug: 269577299
Test: m nothing
Change-Id: I848bff33e4f5798f51296ea3a2600615cab36985
