Fix extension detection

If the path leading up to the Python installation's `site-packages` contains a `.`, this check does not do what it says it does :(

For instance, on macOS, the `site-packages` directory is located at `/usr/local/lib/python3.7/site-packages`, which means that trying to use the magic `%run -m my.module` will always fail, because `split_path[1]` will always start with `7/site-packages`.

There are better ways to check that a file extension matches expectations, but I thought it was cute that I could fix this bug by inserting a single character :)

My current workaround looks like this:
```python
import importlib
import_path = importlib.util.find_spec('my.module').origin
%run $import_path
```

IPython/utils/module_paths.py

@@ -1,70 +1,70 b''
 """Utility functions for finding modules
 
 Utility functions for finding modules on sys.path.
 
 `find_module` returns a path to module or None, given certain conditions.
 
 """
 #-----------------------------------------------------------------------------
 # Copyright (c) 2011, the IPython Development Team.
 #
 # Distributed under the terms of the Modified BSD License.
 #
 # The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 # Stdlib imports
 import importlib
 import os
 
 # Third-party imports
 
 # Our own imports
 
 
 #-----------------------------------------------------------------------------
 # Globals and constants
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Local utilities
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Classes and functions
 #-----------------------------------------------------------------------------
 
 def find_mod(module_name):
     """
     Find module `module_name` on sys.path, and return the path to module `module_name`.
 
       - If `module_name` refers to a module directory, then return path to __init__ file. 
         - If `module_name` is a directory without an __init__file, return None.
       - If module is missing or does not have a `.py` or `.pyw` extension, return None.
         - Note that we are not interested in running bytecode.
       - Otherwise, return the fill path of the module.
 
     Parameters
     ----------
     module_name : str
     
     Returns
     -------
     module_path : str
         Path to module `module_name`, its __init__.py, or None,
         depending on above conditions.
     """
     loader = importlib.util.find_spec(module_name)
     module_path = loader.origin
     if module_path is None:
         return None
     else:
         split_path = module_path.split(".")
-        if split_path[1] in ["py", "pyw"]:
+        if split_path[-1] in ["py", "pyw"]:
             return module_path
         else:
             return None