Reorganize code

[abhinav]

This moves all executables to cmd/ and library code to the top-level.

Resolves #5

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Note that this will break existing callers that do,

go get go.uber.org/tools/{update-license,parallel-exec}

To ensure that nobody breaks, we can,

• Move their main() implementations into internal packages which
  export a Main() function. This function will be called from both,
  the new and old executable paths.
• Alternatively, we can put placeholders there that just fail with a
  message indicating that the new path should be used.

Thoughts?

[abhinav]

Stale. Abandoning.