How should group_imports handle visibility?

1. What should be the default behavior with how imports are grouped by visibility?
   1. Should pub items be sorted differently at all?
   2. Should pub(<visibility>) be sorted differently than pub?
   3. Sort by visibility then by name or just by name? What about pub(in a::b)?
   4. Should pub(<visibility>) cause more blank-line-separated groups? (e.g. a pub group and a pub(crate) group, etc.) or just one group for all pub items?
2. What config options should be available?

No files, tests, or entry points are named in the issue. First resolve the proposed visibility ordering, grouping, and configuration behavior, then identify the relevant rustfmt implementation and tests; the work is done when the decisions are implemented and covered by tests.