[core] Deprecate BlackBox in favor of ExtModule

[seldridge]

Deprecate BlackBox and tell users to switch to ExtModule. BlackBox has a
ton of baggage associated with it, due to it mandating, at runtime, a
single val io call and no additional calls to IO while also magically
removing the io_ prefix in the generated FIRRTL. ExtModule is far
cleaner and works as a user would expect. You can call IO as much as
you want. No magic prefixing happens.

This is slightly unfortunate as one could argue that BlackBox is a good
name. However, you can also argue (more strongly I would add) that this
is an "external module" and it would be better to call it as such. If
this is controversial, we can consider a migration back from ExtModule
to BlackBox.

Migrate some tests that were only testing blackboxes to be duplicated in
the external module tests. This will allow us to just delete this test
later.

This has three motivations:

1. BlackBox has always been janky for the reasons above and ExtModule
   is the saner generator API.

2. It's bad to have two similar, though subtly different ways of doing
   things.

3. The singular IO that BlackBox forces users to go through means that
   BlackBox cannot have domain information attached to it. This
   essentially means that BlackBox will be essentially unusable until
   domains gain support for being in subfields. It's easier to just
   deprecate BlackBox than to require that domains gain this complicated
   feature addition just now.

This is stacked on #5102.

Release Notes

Deprecate BlackBox in favor of ExtModule.