Fix PathEntryFinder / MetaPathFinder in editable_wheel

It seems that the import machinery skips PathEntryFinder when trying to
locate nested namespaces, if the `sys.path_hook` item corresponding to
the finder cannot be located in the submodule locations of the parent
namespace.

This means that we should probably always add the PATH_PLACEHOLDER to
the namespace spec.

This PR also add some type hints to the template, because it helped to
debug some type errors.

Author: abravalheri

setuptools/command/editable_wheel.py

@@ -793,23 +793,22 @@ def _get_root(self):
 
 
 _FINDER_TEMPLATE = """\
+from __future__ import annotations
 import sys
 from importlib.machinery import ModuleSpec, PathFinder
 from importlib.machinery import all_suffixes as module_suffixes
 from importlib.util import spec_from_file_location
 from itertools import chain
 from pathlib import Path
 
-MAPPING = {mapping!r}
-NAMESPACES = {namespaces!r}
+MAPPING: dict[str, str] = {mapping!r}
+NAMESPACES: dict[str, list[str]] = {namespaces!r}
 PATH_PLACEHOLDER = {name!r} + ".__path_hook__"
 
 
 class _EditableFinder:  # MetaPathFinder
     @classmethod
-    def find_spec(cls, fullname, path=None, target=None):
-        extra_path = []
-
+    def find_spec(cls, fullname: str, _path=None, _target=None) -> ModuleSpec | None:
         # Top-level packages and modules (we know these exist in the FS)
         if fullname in MAPPING:
             pkg_path = MAPPING[fullname]
@@ -820,43 +819,50 @@ def find_spec(cls, fullname, path=None, target=None):
         # to the importlib.machinery implementation.
         parent, _, child = fullname.rpartition(".")
         if parent and parent in MAPPING:
-            return PathFinder.find_spec(fullname, path=[MAPPING[parent], *extra_path])
+            return PathFinder.find_spec(fullname, path=[MAPPING[parent]])
 
         # Other levels of nesting should be handled automatically by importlib
         # using the parent path.
         return None
 
     @classmethod
-    def _find_spec(cls, fullname, candidate_path):
+    def _find_spec(cls, fullname: str, candidate_path: Path) -> ModuleSpec | None:
         init = candidate_path / "__init__.py"
         candidates = (candidate_path.with_suffix(x) for x in module_suffixes())
         for candidate in chain([init], candidates):
             if candidate.exists():
                 return spec_from_file_location(fullname, candidate)
+        return None
 
 
 class _EditableNamespaceFinder:  # PathEntryFinder
     @classmethod
-    def _path_hook(cls, path):
+    def _path_hook(cls, path) -> type[_EditableNamespaceFinder]:
         if path == PATH_PLACEHOLDER:
             return cls
         raise ImportError
 
     @classmethod
-    def _paths(cls, fullname):
-        # Ensure __path__ is not empty for the spec to be considered a namespace.
-        return NAMESPACES[fullname] or MAPPING.get(fullname) or [PATH_PLACEHOLDER]
+    def _paths(cls, fullname: str) -> list[str]:
+        paths = NAMESPACES[fullname]
+        if not paths and fullname in MAPPING:
+            paths = [MAPPING[fullname]]
+        # Always add placeholder, for 2 reasons:
+        # 1. __path__ cannot be empty for the spec to be considered namespace.
+        # 2. In the case of nested namespaces, we need to force
+        #    import machinery to query _EditableNamespaceFinder again.
+        return [*paths, PATH_PLACEHOLDER]
 
     @classmethod
-    def find_spec(cls, fullname, target=None):
+    def find_spec(cls, fullname: str, _target=None) -> ModuleSpec | None:
         if fullname in NAMESPACES:
             spec = ModuleSpec(fullname, None, is_package=True)
             spec.submodule_search_locations = cls._paths(fullname)
             return spec
         return None
 
     @classmethod
-    def find_module(cls, fullname):
+    def find_module(cls, _fullname) -> None:
         return None