add mus-ignore-yosys-internal-netnames option

[rperoutka]

Add an option for the MUS engine to not include constraints associated with yosys internal netnames (those with '$'-prefixed names) in the MUS constraint set.

[Isweet]

This looks great, I left a couple minor comments.

The only high-level thought I have is that it might be nice to add clarity somewhere (I'm not sure where it belongs) about what is meant by an 'internal' name. I think even three sentences like the following could be very helpful to future people:

This engine is augmented to map SMT identifiers in the MUS's back to source-level RTL identifiers (e.g., wires and registers in SystemVerilog). During synthesis, Yosys creates additional identifiers, prefixed by $, that do not have a source-level correspondent. We refer to these identifers as 'internal.'

It might seem obvious and verbose now, but could be valuable for future people trying to understand the code.

[Isweet]

Looks great, thanks for addressing the previous small bits of feedback and good catch on the flag issue.