Tweak privacy errors to account for reachable items

Suggest publicly accessible paths for items in private mod:

  When encountering a path in non-import situations that are not reachable
  due to privacy constraints, search for any public re-exports that the
  user could use instead.

Track whether an import suggestion is offering a re-export.

When encountering a path with private segments, mention if the item at
the final path segment is not publicly accessible at all.

Add item visibility metadata to privacy errors from imports:

  On unreachable imports, record the item that was being imported in order
  to suggest publicly available re-exports or to be explicit that the item
  is not available publicly from any path.

  In order to allow this, we add a mode to `resolve_path` that will not
  add new privacy errors, nor return early if it encounters one. This way
  we can get the `Res` corresponding to the final item in the import,
  which is used in the privacy error machinery.

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