`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>

lib/spack/spack/test/versions.py

@@ -607,6 +607,9 @@ def test_stringify_version(version_str):
     v.string = None
     assert str(v) == version_str
 
+    v.string = None
+    assert v.string == version_str
+
 
 def test_len():
     a = Version("1.2.3.4")

lib/spack/spack/version/__init__.py

@@ -25,11 +25,13 @@
 )
 from .version_types import (
     ClosedOpenRange,
+    ConcreteVersion,
     GitVersion,
     StandardVersion,
     Version,
     VersionList,
     VersionRange,
+    VersionType,
     _next_version,
     _prev_version,
     from_string,
@@ -40,21 +42,23 @@
 any_version: VersionList = VersionList([":"])
 
 __all__ = [
-    "Version",
-    "VersionRange",
-    "ver",
-    "from_string",
-    "is_git_version",
-    "infinity_versions",
-    "_prev_version",
-    "_next_version",
-    "VersionList",
     "ClosedOpenRange",
-    "StandardVersion",
+    "ConcreteVersion",
+    "EmptyRangeError",
     "GitVersion",
-    "VersionError",
+    "StandardVersion",
+    "Version",
     "VersionChecksumError",
+    "VersionError",
+    "VersionList",
     "VersionLookupError",
-    "EmptyRangeError",
+    "VersionRange",
+    "VersionType",
+    "_next_version",
+    "_prev_version",
     "any_version",
+    "from_string",
+    "infinity_versions",
+    "is_git_version",
+    "ver",
 ]