break the dependency from rustc_target -> rustc_fs_util

[jyn514]

fs_util is now first needed by rustc_session, so it's not needed before compiling rustc_hir. i haven't yet collected before/after timings, i can do so if you like.

inspired by #160336.

before:

after:

• I used an LLM to create a change in this PR, and I have explained below how it was used.

disclaimer: this PR was LLM generated. i told it to look at #160336 and find similar improvements, and it wrote the code. originally it also tried to break rustc_feature -> rustc_hir and rustc_metadata -> rustc_incremental, but it turns out i was on an old branch and those had already been split up on main.

r? @jackh726 (not the PR i said you would be reviewing, oops, but still quite small)

@rustbot label llm-assisted

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[mejrs]

I'd rather not make this change, rustc_fs_util is compiled way, way before any other rustc crates using it so it's not going to matter for compiling in parallel. I think it's more valuable to have all of rustc's path handling logic being in one place.

[jyn514]

makes sense 👍