BlackBoxUtils use a BlackBox self-type, should be BaseBlackBox?

You currently can't mixin `HasBlackBoxResource` into `ExtModule` due to the self-typing used for `BlackBoxUtils` traits.

**Type of issue**: bug report

**Impact**: API addition (no impact on existing code)

**Development Phase**: proposal

**What is the use case for changing the behavior?**

Being able to use `BlackBoxUtils` annotators for `ExtModule`.

Start by tracing the definitions and self-types of BlackBoxUtils, HasBlackBoxResource, ExtModule, and BaseBlackBox in the Chisel source. Confirm the intended API relationship and verify that HasBlackBoxResource can be mixed into ExtModule without breaking existing BlackBox use cases; the issue names no test or file to run.

Written by the indexing model from the issue text.