cask/audit: audit for appropriate sharding directory

[razvanazamfirei]

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This PR adds an audit to ensure casks are in the appropriate subdirectory.
Should help address the issue raised in Homebrew/homebrew-cask#152603

[MikeMcQuaid]

Should be fine as-is but duplicating some logic that the suggestion should handle.

[MikeMcQuaid]

Thanks @razvanazamfirei! Anyone: feel free to merge when comment addressed.