Remove spack.target from code (#46503)

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>

lib/spack/docs/conf.py

@@ -219,6 +219,7 @@ def setup(sphinx):
     ("py:class", "spack.install_test.Pb"),
     ("py:class", "spack.filesystem_view.SimpleFilesystemView"),
     ("py:class", "spack.traverse.EdgeAndDepth"),
+    ("py:class", "archspec.cpu.microarchitecture.Microarchitecture"),
 ]
 
 # The reST default role (used for this markup: `text`) to use for all documents.