Refine how we detect namespace packages

Previously we used a hand crafted approach to detect namespace packages, however we should rely on ``importlib`` to detect them for us.

Fix pytest-dev#12112

changelog/12112.bugfix.rst

@@ -0,0 +1 @@
+Refine how namespace packages are detected when :confval:`consider_namespace_packages` is enabled.

src/_pytest/pathlib.py

@@ -773,17 +773,14 @@ def resolve_pkg_root_and_module_name(
         pkg_root = pkg_path.parent
         # https://packaging.python.org/en/latest/guides/packaging-namespace-packages/
         if consider_namespace_packages:
-            # Go upwards in the hierarchy, if we find a parent path included
-            # in sys.path, it means the package found by resolve_package_path()
-            # actually belongs to a namespace package.
-            for parent in pkg_root.parents:
+            for candidate in (pkg_root, *pkg_root.parents):
                 # If any of the parent paths has a __init__.py, it means it is not
                 # a namespace package (see the docs linked above).
-                if (parent / "__init__.py").is_file():
+                if (candidate / "__init__.py").is_file():
                     break
-                if str(parent) in sys.path:
+                if _is_namespace_package(candidate):
                     # Point the pkg_root to the root of the namespace package.
-                    pkg_root = parent
+                    pkg_root = candidate.parent
                     break
 
         names = list(path.with_suffix("").relative_to(pkg_root).parts)
@@ -795,6 +792,29 @@ def resolve_pkg_root_and_module_name(
     raise CouldNotResolvePathError(f"Could not resolve for {path}")
 
 
+def _is_namespace_package(module_path: Path) -> bool:
+    module_name = module_path.name
+
+    # Empty module names break find_spec.
+    if not module_name:
+        return False
+
+    # Modules starting with "." indicate relative imports and break find_spec, and we are only attempting
+    # to find top-level namespace packages anyway.
+    if module_name.startswith("."):
+        return False
+
+    spec = importlib.util.find_spec(module_name)
+    if spec is not None and spec.submodule_search_locations:
+        # Found a spec, however make sure the module_path is in one of the search locations --
+        # this ensures common module name like "src" (which might be in sys.path under different locations)
+        # is only considered for the module_path we intend to.
+        # Make sure to compare Path(s) instead of strings, as this will normalize them on Windows.
+        if module_path in [Path(x) for x in spec.submodule_search_locations]:
+            return True
+    return False
+
+
 class CouldNotResolvePathError(Exception):
     """Custom exception raised by resolve_pkg_root_and_module_name."""