fix: avoid prelude paths when imports.preferPrelude is false

[Souradip121]

What changes

• Rename Choice::prefer_due_to_prelude → has_prelude_segment; track it
  unconditionally instead of gating on the config value
• Make Choice::try_select bidirectional: deprioritize prelude paths when
  prefer_prelude: false, prefer them when true
• Add container_max_len helper that relaxes the path-search limit by 1 when
  the current best is a prelude path and prefer_prelude: false, so a canonical
  path one segment longer is not silently cut off before it can be considered

Before / After

// Crate exposes two paths to the same item:
//   krate::prelude::Foo          (3 segments — re-export)
//   krate::module::sub::Foo      (4 segments — canonical)

// Before  (prefer_prelude: false, the default)
use krate::prelude::Foo;        // shorter path always won

// After   (prefer_prelude: false, the default)
use krate::module::sub::Foo;   // canonical path is preferred

// After   (prefer_prelude: true)
use krate::prelude::Foo;        // prelude path preferred, unchanged

Notes

• No new config key — rust-analyzer.imports.preferPrelude: false (the default)
  now has correct semantics in both directions
• find_in_prelude early-return (Rust language prelude items already in scope)
  is intentionally untouched; this fix only affects ranked path selection
• Closes Option for import path when re-exports exist — preferences/excludes #21914

Used AI for faster development, but properly reviewed the code myself after writing it.

[ChayimFriedman2]

Note that we require disclosure of AI tools, if you used them. See Use of AI tools in CONTRIBUTING.md.

[Souradip121]

@ChayimFriedman2 Yep, used AI, already disclosed in the commit message.

[ChayimFriedman2]

The current code passing the best path so far's length as max_len is wrong, see discussion on Zulip. Instead, we should pass the current max_len minus 1, and this will also solve the need for container_max_len. You can do that in this PR or in a separate PR.

[Souradip121]

@ChayimFriedman2 doing in this PR only.

[Souradip121]

@ChayimFriedman2 check once!

[ChayimFriedman2]

Thanks!

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