enum type instead of variant suggestion unification

Weirdly, we were deciding between a help note and a structured
suggestion based on whether the import candidate span was a dummy—but
we weren't using that span in any case! The dummy-ness of the span
(which appears to be a matter of this-crate vs. other-crate
definition) isn't the right criterion by which we should decide
whether it's germane to mention that "there is an enum variant";
instead, let's use the someness of `def` (which is used as the
`has_unexpected_resolution` argument to `error_code`).

Since `import_candidate_to_paths` has no other callers, we are free to
stop returning the span and rename the function. By using
`span_suggestions_`, we leverage the max-suggestions output limit
already built in to the emitter, thus resolving #56028.

In the matter of message wording, "you can" is redundant (and perhaps
too informal); prefer the imperative.

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