Fill-in missing implementation for is_native_module_file_name (#5410)

## Summary

This was just an oversight -- the last remaining `todo!()` that I never
filled in. We clearly don't have any test coverage for it yet, but this
mimics the Pyright implementation.

crates/ruff_python_resolver/src/native_module.rs

@@ -8,7 +8,66 @@ pub(crate) fn is_native_module_file_extension(file_extension: &OsStr) -> bool {
     file_extension == "so" || file_extension == "pyd" || file_extension == "dylib"
 }
 
-/// Returns `true` if the given file name is that of a native module.
-pub(crate) fn is_native_module_file_name(_module_name: &Path, _file_name: &Path) -> bool {
-    todo!()
+/// Given a file name, returns the name of the native module it represents.
+///
+/// For example, given `foo.abi3.so`, return `foo`.
+pub(crate) fn native_module_name(file_name: &Path) -> Option<&str> {
+    file_name
+        .file_stem()
+        .and_then(OsStr::to_str)
+        .map(|file_stem| {
+            file_stem
+                .split_once('.')
+                .map_or(file_stem, |(file_stem, _)| file_stem)
+        })
+}
+
+/// Returns `true` if the given file name is that of a native module with the given name.
+pub(crate) fn is_native_module_file_name(module_name: &str, file_name: &Path) -> bool {
+    // The file name must be that of a native module.
+    if !file_name
+        .extension()
+        .map_or(false, is_native_module_file_extension)
+    {
+        return false;
+    };
+
+    // The file must represent the module name.
+    native_module_name(file_name) == Some(module_name)
+}
+
+#[cfg(test)]
+mod tests {
+    use std::path::PathBuf;
+
+    #[test]
+    fn module_name() {
+        assert_eq!(
+            super::native_module_name(&PathBuf::from("foo.so")),
+            Some("foo")
+        );
+
+        assert_eq!(
+            super::native_module_name(&PathBuf::from("foo.abi3.so")),
+            Some("foo")
+        );
+
+        assert_eq!(
+            super::native_module_name(&PathBuf::from("foo.cpython-38-x86_64-linux-gnu.so")),
+            Some("foo")
+        );
+
+        assert_eq!(
+            super::native_module_name(&PathBuf::from("foo.cp39-win_amd64.pyd")),
+            Some("foo")
+        );
+    }
+
+    #[test]
+    fn module_file_extension() {
+        assert!(super::is_native_module_file_extension("so".as_ref()));
+        assert!(super::is_native_module_file_extension("pyd".as_ref()));
+        assert!(super::is_native_module_file_extension("dylib".as_ref()));
+        assert!(!super::is_native_module_file_extension("py".as_ref()));
+    }
 }