Fix skip dir inventory extensions

I discovered this after I upgraded from 1.6 to the current devel branch. My /etc/ansible looks like this:
```
├── ansible.cfg
└── hosts
    ├── ec2
    ├── ec2.ini
    └── local
```
(where ec2 is really ec2.py)

And doing e.g. `ansible --list-hosts all` started returning ini errors. I discovered that it was because the extension filtering broke in a recent refactoring of ansible.inventory.dir.InventoryDirectory.

This pull request includes a new test that demonstrates the issue as well as a fix.