Fix pyclbr to support importing packages

Issue #26569: Fix pyclbr.readmodule() and pyclbr.readmodule_ex() to support
importing packages.
diff --git a/Lib/pyclbr.py b/Lib/pyclbr.py
index dd58ada..4d40b87 100644
--- a/Lib/pyclbr.py
+++ b/Lib/pyclbr.py
@@ -142,10 +142,10 @@
         search_path = path + sys.path
     # XXX This will change once issue19944 lands.
     spec = importlib.util._find_spec_from_path(fullmodule, search_path)
-    fname = spec.loader.get_filename(fullmodule)
     _modules[fullmodule] = dict
-    if spec.loader.is_package(fullmodule):
-        dict['__path__'] = [os.path.dirname(fname)]
+    # is module a package?
+    if spec.submodule_search_locations is not None:
+        dict['__path__'] = spec.submodule_search_locations
     try:
         source = spec.loader.get_source(fullmodule)
         if source is None:
@@ -154,6 +154,8 @@
         # not Python source, can't do anything with this module
         return dict
 
+    fname = spec.loader.get_filename(fullmodule)
+
     f = io.StringIO(source)
 
     stack = [] # stack of (class, indent) pairs