Rust: Fix bad join

[hvitved]

Evaluated relational algebra for predicate _Crate::Crate.getSourceFile/0#dispred#e7adf9d7_Crate::Generated::Crate.getName/0#dispred#f4d3b3bf_Pa__#join_rhs@5a04a7t0 with tuple counts:
        34471980   ~0%    {3} r1 = JOIN `PathResolution::isSourceFile/1#803de032` WITH `Crate::Crate.getSourceFile/0#dispred#e7adf9d7` CARTESIAN PRODUCT OUTPUT Rhs.0, Lhs.0, Rhs.1
        34471980  ~37%    {4}    | JOIN WITH `Crate::Generated::Crate.getName/0#dispred#f4d3b3bf` ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Rhs.1, _
                          {3}    | REWRITE WITH NOT [Tmp.3 := "std", TEST InOut.2 != Tmp.3, Tmp.3 := "core", TEST InOut.2 != Tmp.3] KEEPING 3
           93420  ~91%    {3}    | SCAN OUTPUT In.1, _, In.0
           93420  ~87%    {3}    | REWRITE WITH Out.1 := "prelude"
                          return r1

[Copilot]

Pull Request Overview

This PR optimizes a performance bottleneck in the Rust query language code by refactoring duplicate predicate logic to improve join efficiency. The change addresses a bad join pattern that was causing excessive tuple counts during query evaluation.

• Extracted shared logic for finding prelude items into a separate predicate
• Reordered join conditions in preludeEdge to optimize query performance

[geoffw0]

LGTM, thanks for fixing.