The default cli Warn calls always write to the error writer (stderr by
default), however the output is intended to be viewed in the UI by the
user, rather than in a separate stream. Terraform also generally does
not consider warnings to be errors from the cli point of view, and does
not need to output the warning text to stderr.

By redirecting Warn calls to Output calls at the lowest level in the
main package, we can eliminate the chance that Warn and Output
messages are interleaved, while still allowing the internal `cli.Ui`
implementations to format `Warn` and `Output` calls separately.