`PackagePrefs` has had a class-level cache of data from `packages.yaml` for
a long time, but it complicates testing and leads to subtle errors,
especially now that we frequently manipulate custom config scopes and
environments.
Moving the cache to instance-level doesn't slow down concretization or
the test suite, and it just caches for the life of a `PackagePrefs`
instance (i.e., for a single cocncretization) so we don't need to worry
about global state anymore.
- [x] Remove class-level caches from `PackagePrefs`
- [x] Add a cached _spec_order object on each `PackagePrefs` instance
- [x] Remove all calls to `PackagePrefs.clear_caches()`
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