Remove AST checker's dependency on resolver (#3368)

crates/ruff_python_stdlib/src/path.rs

@@ -0,0 +1,34 @@
+use std::path::Path;
+
+/// Return `true` if the [`Path`] appears to be that of a Python file.
+pub fn is_python_file(path: &Path) -> bool {
+    path.extension()
+        .map_or(false, |ext| ext == "py" || ext == "pyi")
+}
+
+/// Return `true` if the [`Path`] appears to be that of a Python interface definition file (`.pyi`).
+pub fn is_python_stub_file(path: &Path) -> bool {
+    path.extension().map_or(false, |ext| ext == "pyi")
+}
+
+#[cfg(test)]
+mod tests {
+    use std::path::Path;
+
+    use crate::path::is_python_file;
+
+    #[test]
+    fn inclusions() {
+        let path = Path::new("foo/bar/baz.py");
+        assert!(is_python_file(path));
+
+        let path = Path::new("foo/bar/baz.pyi");
+        assert!(is_python_file(path));
+
+        let path = Path::new("foo/bar/baz.js");
+        assert!(!is_python_file(path));
+
+        let path = Path::new("foo/bar/baz");
+        assert!(!is_python_file(path));
+    }
+}