Search for all python3.x in PATH (#5148)

Search for all `python3.x` minor versions in PATH, skipping those we
already know we can use.

For example, let's say `python` and `python3` are Python 3.10. When a
user requests `>= 3.11`, we still need to find a `python3.12` in PATH.
We do so with a regex matcher.

Fixes #4709

crates/uv-python/src/which.rs

@@ -0,0 +1,40 @@
+use std::path::Path;
+
+/// Check whether a path in PATH is a valid executable.
+///
+/// Derived from `which`'s `Checker`.
+pub(crate) fn is_executable(path: &Path) -> bool {
+    #[cfg(any(unix, target_os = "wasi", target_os = "redox"))]
+    {
+        if rustix::fs::access(path, rustix::fs::Access::EXEC_OK).is_err() {
+            return false;
+        }
+    }
+
+    #[cfg(target_os = "windows")]
+    {
+        let Ok(file_type) = fs_err::symlink_metadata(path).map(|metadata| metadata.file_type())
+        else {
+            return false;
+        };
+        if !file_type.is_file() && !file_type.is_symlink() {
+            return false;
+        }
+        if path.extension().is_none()
+            && winsafe::GetBinaryType(&path.display().to_string()).is_err()
+        {
+            return false;
+        }
+    }
+
+    if cfg!(not(target_os = "windows")) {
+        if !fs_err::metadata(path)
+            .map(|metadata| metadata.is_file())
+            .unwrap_or(false)
+        {
+            return false;
+        }
+    }
+
+    true
+}