fix: emit visible re-export path when canonical traverses a private module

[swjng]

Summary

redundant_closure_for_method_calls was emitting suggestions built from
tcx.def_path directly. When the canonical path traversed a private
module, the suggestion did not compile (E0603), and clippy --fix
replaced working code with broken code.

Closes #16731.

changelog: [redundant_closure_for_method_calls]: avoid suggesting paths that go through a private module

Approach

In get_path_to_callee, three tiers:

1. Short relative path when every segment of the canonical path is
   accessible from the caller.
2. Public re-export path otherwise — BFS the local crate from the
   root for the shortest visible path. Skips pub use _ as _ and
   #[doc(hidden)]; descends through Mod | Enum | Trait.
3. Canonical name with Applicability::MaybeIncorrect if neither
   works — the diagnostic still shows, but --fix no longer
   auto-applies it.

get_path_from_caller_to_method_type now returns
(String, Applicability); the single caller in eta_reduction.rs is
updated.

Test plan

• Regression cases in tests/ui/eta.rs (mod issue16731) covering
  inherent and trait methods re-exported across a private module.
• cargo test --test compile-test (1759 + 188 + 46 tests) clean.
• cargo test --test dogfood, cargo fmt --check clean.

---

AI tools used: Claude Code. All changes reviewed and tested by the author.

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[github-actions]

Lintcheck changes for 23bec31

Lint	Added	Removed	Changed
clippy::redundant_closure_for_method_calls	0	0	15

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

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