The `spack.target.Target` class is a weird entity, that is just needed to:
1. Sort microarchitectures in lists deterministically
2. Being able to use microarchitectures in hashed containers
This PR removes it, and uses `archspec.cpu.Microarchitecture` directly. To sort lists, we use a proper `key=` when needed. Being able to use `Microarchitecture` objects in sets is achieved by updating the external `archspec`.
Signed-off-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>
b93c57cab9