rustc_public: split defs out of ty

[makai410]

This is more of an "I feel like this should be like this" kind of change. I suppose that we might need to make the ty a bit more fine-grained as the coverage expands. That said, I don't have a strong opinion on whether it's necessary, so feel free to close this PR if you find it not convincing enough.

The diff looks so scary, though... I just split ty.rs into def.rs and tys.rs and changed crate_def! and crate_def_with_ty!.

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

[rustbot]

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a

[rustbot]

r? @scottmcm

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