fix: rename rustfmt test modules to avoid case-insensitive path collisions

[cataggar]

Summary

Cloning the rust repo on case-insensitive filesystems (e.g. Windows, macOS) produces a git warning about colliding paths:

warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'src/tools/rustfmt/tests/source/reorder_modules/ABCD/mod.rs'
  'src/tools/rustfmt/tests/source/reorder_modules/abcd/mod.rs'
  'src/tools/rustfmt/tests/source/reorder_modules/ZYXW/mod.rs'
  'src/tools/rustfmt/tests/source/reorder_modules/zyxw/mod.rs'
  'src/tools/rustfmt/tests/target/reorder_modules/ABCD/mod.rs'
  'src/tools/rustfmt/tests/target/reorder_modules/abcd/mod.rs'
  'src/tools/rustfmt/tests/target/reorder_modules/ZYXW/mod.rs'
  'src/tools/rustfmt/tests/target/reorder_modules/zyxw/mod.rs'

Why they were named this way

These were added by @calebcartwright in 2d049af8f0e (rust-lang/rustfmt#6368) and brought into this repo via the rustfmt subtree update (rust-lang#153145). The ABCD/abcd and ZYXW/zyxw directories test case-sensitive module sorting — Edition ≤ 2021 uses str::cmp (ASCII order) while Edition ≥ 2024 uses version_sort. The collision on case-insensitive filesystems was not considered since development was on Linux.

Fix

Rename the lowercase directory variants to avoid the collision:

• abcd → abcde
• zyxw → zyxwv

All mod abcd; / mod zyxw; declarations in the 10 affected test files are updated accordingly. This preserves the test intent (verifying case-sensitive module sorting across style editions) while eliminating the warning on case-insensitive filesystems.

Fixes #1

Upstream PR: rust-lang#154639