Allow package_data files to not have an extension

[jochenparm]

Fix for the following issue:
#14

[jochenparm]

Hello @orsinium
This is the minimal change to a relatively small problem which is currently completely blocking our adoption of DepHell in my development team.
I hope we can quickly merge this in, as I'd like to continue our adoption of DepHell as otherwise we're going to have to switch to a different solution as it's quite a pressing issue for us.
If there's anything that needs to be done still for you to merge this in, let me know and I'll invest the time to do so ASAP!

[jochenparm]

@orsinium
Hey, is there a reason you're not mergin in my PR?
My team's adoption of dephell is still blocked by this issue. The lack of response from your side is frustrating, you tell me to create a PR when I file an issue ticket and when I do there's no more communication. If there's a reason why you don't want to merge it in then please tell me and we can find a solution that everyone is happy with. The actual fix is less than one line of code, so it won't take a lot of time to review.
I can remove all the CI related changes in this PR if you prefer, I just implemented that to be able to get a passing run.
The actual change I need is for dephell to no longer be opinionated about whether files need extensions in order to be discoverable. If you see a problem with that, let me know.

[jochenparm]

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