Refactor into subpackage to allow importing

[elmeyer]

Thank you very much for this well-designed, useful and not least extremely comprehensive package!

I am opening this pull request to allow this package to be imported in and used by other Go packages. To this end, I added a go.mod+go.sum to this package. Otherwise (thankfully), the exported functions for verification were already perfectly suitable for this, so only minimal adjustments were required.

I simply split and renamed the functions responsible for determining the Windows version into a subpackage named winver, purely for semantic reasons, and moved most of the remaining code into the verify subpackage. No other changes were made.

Please let me know if I missed something or if you'd like to see any changes. I look forward to your feedback!

Should you decide to accept this pull request, please remember to remove the replace directive in go.mod when merging.