Branched from #12071:

Currently, the filegroup rule doesn't participate in toolchain resolution (see https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/bazel/rules/common/BazelFilegroupRule.java;l=78), which means the target platform doesn't exist and so platform-conditional selects don't work.

As I recall, it led to a cycle in evaluation, because filegroups were needed at a low-enough level that we couldn't process loading platforms without them.

I'll investigate whether this is still an issue and whether this can be re-enabled.