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8485dbb3 - [ty] Fix `--python` argument for Windows, and improve error messages for bad `--python` arguments (#18457)

[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