Improved path handling and Windows build fix

[davebalmer]

Improved file handling internally. Moved away from path/filepath in favor of path. Keep paths internally set to / separators to keep everything simple. Go's low level IO already resolves to \ on Windows, so we don't need to worry about it in our code. This should be smoother overall, and fix the breakage in Windows.

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This discussion was created from the release Improved path handling and Windows build fix.