[ty] Fix --python argument for Windows, and improve error messages for bad --python arguments (#18457)

## Summary

Fixes https://github.com/astral-sh/ty/issues/556.

On Windows, system installations have different layouts to virtual
environments. In Windows virtual environments, the Python executable is
found at `<sys.prefix>/Scripts/python.exe`. But in Windows system
installations, the Python executable is found at
`<sys.prefix>/python.exe`. That means that Windows users were able to
point to Python executables inside virtual environments with the
`--python` flag, but they weren't able to point to Python executables
inside system installations.

This PR fixes that issue. It also makes a couple of other changes:
- Nearly all `sys.prefix` resolution is moved inside `site_packages.rs`.
That was the original design of the `site-packages` resolution logic,
but features implemented since the initial implementation have added
some resolution and validation to `resolver.rs` inside the module
resolver. That means that we've ended up with a somewhat confusing code
structure and a situation where several checks are unnecessarily
duplicated between the two modules.
- I noticed that we had quite bad error messages if you e.g. pointed to
a path that didn't exist on disk with `--python` (we just gave a
somewhat impenetrable message saying that we "failed to canonicalize"
the path). I improved the error messages here and added CLI tests for
`--python` and the `environment.python` configuration setting.

## Test Plan

- Existing tests pass
- Added new CLI tests
- I manually checked that virtual-environment discovery still works if
no configuration is given
- Micha did some manual testing to check that pointing `--python` to a
system-installation executable now works on Windows

crates/ty_test/src/lib.rs

@@ -272,7 +272,7 @@ fn run_test(
             python_path: configuration
                 .python()
                 .map(|sys_prefix| {
-                    PythonPath::SysPrefix(
+                    PythonPath::IntoSysPrefix(
                         sys_prefix.to_path_buf(),
                         SysPrefixPathOrigin::PythonCliFlag,
                     )