Add parameter to not filter out blackboxes in selected_modules and selected_whole_modules

[RCoeurjoly]

What are the reasons/motivation for this change?
The select command in Yosys includes an option that allows patterns to match black/white-box modules or their contents by prefixing the pattern with =. This is documented here:

By default, patterns will not match black/white-box modules or their
contents. To include such objects, prefix the pattern with '='.

However, the method RTLIL::Design::selected_modules() filters out blackboxes, so adding a = to a pattern does nothing.

Explain how this is achieved.
We add a include_bw_box parameter that defaults to false to RTLIL::Design::selected_modules() and RTLIL::Design::selected_whole_modules().

If applicable, please suggest to reviewers how they can test the change.

Discarded options

Unconditionally selecting blackboxes

That would break some passes, like opt.

Adding a -blackbox flag to cutpoint

#4566