`version_types`: clean up type hierarchy and add annotations (#47781)

In preparation for adding `when=` to `version()`, I'm cleaning up the types in
`version_types` and making sure the methods here pass `mypy` checks. This started as an
attempt to use `ConcreteVersion` outside of `spack.version` and grew into a larger type
refactor.

The hierarchy now looks like this:

* `VersionType`
  * `ConcreteVersion`
    * `StandardVersion`
    * `GitVersion`
  * `ClosedOpenRange`
  * `VersionList`

Note that the top-level thing can't easily be `Version` as that is a method and it
returns only `ConcreteVersion` right now. I *could* do something fancy with `__new__` to
make `Version` a synonym for the `ConcreteVersion` constructor, which would allow it to
be used as a type. I could also do something similar with `VersionRange` but not sure if
it's worth it just to make these into types.

There are still some places where I think `GitVersion` might not be handled properly,
but I have not attempted to fix those here.

- [x] Add a top-level `VersionType` class that all version types extend from
- [x] Define and document common methods and rich comparisons on `VersionType`
- [x] Replace complicated `Union` types with `VersionType` and `ConcreteVersion` as needed
- [x] Annotate most methods (skipping `__getitem__` and friends as the typing is a pain)
- [x] Fix up the `VersionList` constructor a bit
- [x] Add cases to methods that weren't handling all `VersionType`s
- [x] Rework some places to clarify typing for `mypy`
- [x] Simplify / optimize _next_version
- [x] Make StandardVersion.string a property to enable lazy comparison

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

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