Speed up concretization (#5716)

This isn't a rework of the concretizer but it speeds things up a LOT.

The main culprits were:
  1. Variant code, `provider_index`, and `concretize.py` were calling
     `spec.package` when they could use `spec.package_class`
    - `spec.package` looks up a package instance by `Spec`, which requires a
      (fast-ish but not that fast) DAG compare.
    - `spec.package_class` just looks up the package's class by name, and you
        should use this when all you need is metadata (most of the time).
    - not really clear that the current way packages are looked up is
      necessary -- we can consider refactoring that in the future.

  2. `Repository.repo_for_pkg` parses a `str` argument into a `Spec` when
     called with one, via `@_autospec`, but this is not needed.
     - Add some faster code to handle strings directly and avoid parsing

This speeds up concretization 3-9x in my limited tests.  Still not super
fast but much more bearable:

Before:
  - `spack spec xsdk` took 33.6s
  - `spack spec dealii` took 1m39s

After:
  - `spack spec xsdk` takes 6.8s
  - `spack spec dealii` takes 10.8s

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