[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_python_semantic/src/program.rs

@@ -262,8 +262,10 @@ impl SearchPathSettings {
 #[derive(Debug, Clone, Eq, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum PythonPath {
-    /// A path that represents the value of [`sys.prefix`] at runtime in Python
-    /// for a given Python executable.
+    /// A path that either represents the value of [`sys.prefix`] at runtime in Python
+    /// for a given Python executable, or which represents a path relative to `sys.prefix`
+    /// that we will attempt later to resolve into `sys.prefix`. Exactly which this variant
+    /// represents depends on the [`SysPrefixPathOrigin`] element in the tuple.
     ///
     /// For the case of a virtual environment, where a
     /// Python binary is at `/.venv/bin/python`, `sys.prefix` is the path to
@@ -275,13 +277,7 @@ pub enum PythonPath {
     /// `/opt/homebrew/lib/python3.X/site-packages`.
     ///
     /// [`sys.prefix`]: https://docs.python.org/3/library/sys.html#sys.prefix
-    SysPrefix(SystemPathBuf, SysPrefixPathOrigin),
-
-    /// Resolve a path to an executable (or environment directory) into a usable environment.
-    Resolve(SystemPathBuf, SysPrefixPathOrigin),
-
-    /// Tries to discover a virtual environment in the given path.
-    Discover(SystemPathBuf),
+    IntoSysPrefix(SystemPathBuf, SysPrefixPathOrigin),
 
     /// Resolved site packages paths.
     ///
@@ -291,16 +287,8 @@ pub enum PythonPath {
 }
 
 impl PythonPath {
-    pub fn from_virtual_env_var(path: impl Into<SystemPathBuf>) -> Self {
-        Self::SysPrefix(path.into(), SysPrefixPathOrigin::VirtualEnvVar)
-    }
-
-    pub fn from_conda_prefix_var(path: impl Into<SystemPathBuf>) -> Self {
-        Self::Resolve(path.into(), SysPrefixPathOrigin::CondaPrefixVar)
-    }
-
-    pub fn from_cli_flag(path: SystemPathBuf) -> Self {
-        Self::Resolve(path, SysPrefixPathOrigin::PythonCliFlag)
+    pub fn sys_prefix(path: impl Into<SystemPathBuf>, origin: SysPrefixPathOrigin) -> Self {
+        Self::IntoSysPrefix(path.into(), origin)
     }
 }