Auto merge of #53073 - Mark-Simulacrum:data-structures, r=pnkfelix

Cleanup to librustc::session and related code

No functional changes, just some cleanup.

This also creates the `rustc_fs_util` crate, but I can remove that change if desired. It felt a little odd to force crates to depend on librustc for some fs utilities; and also seemed good to generally keep the size of librustc lower (for compile times); fs_util will compile in parallel with essentially the first crate since it has no dependencies beyond std.

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