Improve error message for missing dependencies

[terror]

Resolves #16499

This diff uses rustc-style diagnostics with help: lines instead of inline semicolon-separated messages when a dependency cannot be found.

Before:

  error: the dependency `fo` could not be found in `dependencies`; dependency `foo` exists

After:

  error: the dependency `fo` could not be found in `dependencies`

  help: a dependency with a similar name exists: `foo`

r? @weihanglo

[rustbot]

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[weihanglo]

Thanks!

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