spack.repo.get() can only be called on concrete specs (#31411)

The goal of this PR is to make clearer where we need a package object in Spack as opposed to a package class.

We currently instantiate a lot of package objects when we could make do with a class.  We should use the class
when we only need metadata, and we should only instantiate and us an instance of `PackageBase` at build time.

Modifications:
- [x] Remove the `spack.repo.get` convenience function (which was used in many places, and not really needed)
- [x] Use `spack.repo.path.get_pkg_class` wherever possible
- [x] Try to route most of the need for `spack.repo.path.get` through `Spec.package`
- [x] Introduce a non-data descriptor, that can be used as a decorator, to have "class level properties"
- [x] Refactor unit tests that had to be modified to reduce code duplication
- [x] `Spec.package` and `Repo.get` now require a concrete spec as input
- [x] Remove `RepoPath.all_packages` and `Repo.all_packages`

var/spack/repos/builtin.mock/packages/multimethod/package.py

@@ -19,6 +19,16 @@ class Multimethod(MultimethodBase):
     homepage = 'http://www.example.com/'
     url      = 'http://www.example.com/example-1.0.tar.gz'
 
+    version('5.0', '0123456789abcdef0123456789abcdef')
+    version('4.0', '0123456789abcdef0123456789abcdef')
+    version('3.0', '0123456789abcdef0123456789abcdef')
+    version('2.0', '0123456789abcdef0123456789abcdef')
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    variant('mpi', default=False, description='')
+
+    depends_on('mpi', when='+mpi')
+
     #
     # These functions are only valid for versions 1, 3, and 4.
     #
@@ -76,9 +86,9 @@ def has_a_default(self):
     def has_a_default(self):
         return 'gcc'
 
-    @when('%intel')
+    @when('%clang')
     def has_a_default(self):
-        return 'intel'
+        return 'clang'
 
     #
     # Make sure we can switch methods on different target