Avoid hidden circular dependencies in spack.architecture (#25873)

* Refactor platform etc. to avoid circular dependencies

All the base classes in spack.architecture have been
moved to the corresponding specialized subpackages,
e.g. Platform is now defined within spack.platforms.

This resolves a circular dependency where spack.architecture
was both:
- Defining the base classes for spack.platforms, etc.
- Collecting derived classes from spack.platforms, etc.
Now it dopes only the latter.

* Move a few platform related functions to "spack.platforms"

* Removed spack.architecture.sys_type()

* Fixup for docs

* Rename Python modules according to review

lib/spack/docs/developer_guide.rst

@@ -211,7 +211,7 @@ Spec-related modules
      yet.
 
 :mod:`spack.architecture`
-  :func:`architecture.sys_type <spack.architecture.sys_type>` is used
+  :func:`architecture.default_arch <spack.architecture.default_arch>` is used
   to determine the host architecture while building.
 
   .. warning::