`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`

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