Rollup merge of #80514 - pietroalbini:fix-install, r=Mark-Simulacrum

Fix broken ./x.py install

During my tarball refactorings in https://github.com/rust-lang/rust/pull/79788 I changed the directory layout used by the tarball generation code, and that broke the other parts of rustbuild which hardcoded the paths of those directories. Namely, `./x.py install` relied on the uncompressed copy of the tarball left behind by `fabricate`/`rust-installer`, causing https://github.com/rust-lang/rust/issues/80494.

While the easy fix for https://github.com/rust-lang/rust/issues/80494 would've been to just update the hardcoded paths to match the new structure, that fix would leave us in the same situation if we were to change the directory layout again in the future. Instead I refactored the code to return a `GeneratedTarball` struct as the output of all the dist steps, and I put all the paths the rest of rustbuild needs to care about in its fields. That way, future changes to `src/bootstrap/tarball.rs` will not break other stuff.

This PR is best reviewed commit-by-commit.
r? `@Mark-Simulacrum`
`@rustbot` modify labels: beta-nominated beta-accepted T-release

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