make hugr optional for QIR emulation

[qartik]

QIR emulation shouldn't require depending on hugr toolchain packages like hugr and selene-hugr-qis-compiler.

The other way applies as well -- HUGR emulation doesn't necessarily require QIR packages like qir_qis.

[jake-arkinstall]

This is a fun architectural one.

At the moment I'm working on decoupling selene from guppylang in dev (as the runtime dep is the inverse - guppylang depends on selene). Ultimately selene's only real input is QIS, and the conversions to that (via that selene-core build system) are provided for ease of use.

I think it would be worthwhile making hugr and qir support independently optional.

I want to keep some 'defaults' when users use selene, so they don't need to use explicit commands to register things with selene, so one approach could be optional dependencies, using conditional importlib guards rather than direct imports.

[qartik]

I think it's fine to default to hugr toolchain. Perhaps we can add "features"/"extras" that correspond to qir-only or hugr-only.