rules_cuda ignores `external_includes` field in compilation context

`external_includes` was added in ~7.0.0 and rules_cuda ignore that field when merging CcInfos, which has the annoying effect that some deps are not made available to the compiler.

This is a placeholder to reference the issue in the code, and maybe propose a fix later.

Locate the rules_cuda code that merges CcInfos and inspect how external_includes is handled. Compare the compilation context behavior with the Bazel version where external_includes was added, then identify a regression test that demonstrates dependencies reaching the compiler. Done means the affected dependencies are available during compilation and the behavior is covered by a test.

Written by the indexing model from the issue text.