modulefinder: make -p handle namespace packages correctly

Fixes part of python#5759
The other part is passing files arguments, which we make steps towards
in python#9614

mypy/modulefinder.py

@@ -387,13 +387,13 @@ def find_modules_recursive(self, module: str) -> List[BuildSource]:
                     if mod not in hits:
                         hits.add(mod)
                         result += self.find_modules_recursive(module + '.' + mod)
-        elif os.path.isdir(module_path) and module in self.ns_packages:
-            # Even more subtler: handle recursive decent into PEP 420
+        elif os.path.isdir(module_path) and any(module.startswith(p) for p in self.ns_packages):
+            # Even subtler: handle recursive decent into PEP 420
             # namespace packages that are explicitly listed on the command
             # line with -p/--packages.
             for item in sorted(self.fscache.listdir(module_path)):
-                if os.path.isdir(os.path.join(module_path, item)):
-                    result += self.find_modules_recursive(module + '.' + item)
+                item, _ = os.path.splitext(item)
+                result += self.find_modules_recursive(module + '.' + item)
         return result