Fix support for additional modules when these are single file modules

For example, to generate a thin.tgz tarball that is able to use dockerio salt
state, you would do:
```
>>> salt.utils.thin.gen_thin('/tmp', 'msgpack,docker,websocket,six,requests', True)
```
But it would add a wrong path to the six.py module in the thin.tgz tarball
because six.py is a single file python module.
This pull request fixes this.

(refs #9337)
Merge pull request #9562 from bbinet/salt-thin-singlefile-module-support

Fix support for additional modules when these are single file modules