Useful error message if somebody passes a filename instead of module name to stub/apply

[carljm]

We should be able to detect this with a heuristic like "no traces found and os.path.exists(...)"

Shouldn't just use a naive endswith('.py'), it's perfectly valid to have a package or module named py.

[carljm]

#37 partially implemented this, but since it looks for os.sep, it doesn't cover the case where someone has a top-level module and just does monkeytype stub mymodule.py. So it could still be improved somewhat by doing the os.path.exists check in case of no traces found.

[piperchester]

It looks like this is in progress by @iamrajhans ? I'm looking for a good first issue to get involved as well 😄