Spack issues tracker

Spack package issues to resolve. Fixes to be made in `spack/spack-packages` but tracking here.

- [ ] JIT via `py-cffi` requires `py-setuptools`. Should this go into the FFCx and/or DOLFINx spec?
- [x] Remove the Boost `+filesystem+program_options+timer` options (the add-ons are no longer used by DOLFINx).
- [x] For `fenics-dolfinx`, PETSc and SLEPc are optional (disabled by default, which is good Spack practice), but `py-petsc4py` is a dependency for `py-fenics-dolfix`. This means we can get the Python interface with petsc4py but the backend without PETSc (similarly for SLEPc).
- [x] No `py-slepc4py` option for `py-fenics-dolfix`.
- [ ] Add `no_cache=True` to `version` for development versions (all FEniCS packages).
- [ ] Add test dependencies
- [ ] Add CMake 'Developer' build type
- [ ] Deprecate older versions.