Rollup merge of #147506 - Zalathar:isolate, r=jieyouxu

compiletest: Isolate public APIs and minimize public surface area

As part of my ongoing efforts to improve directive parsing, I would like to be able to make internal changes without worrying about whether they're going to break the rustdoc-gui-test tool. That tool currently uses compiletest as a dependency to help with directive parsing.

This PR therefore isolates all of compiletest's public APIs into two dedicated modules, one used by rustdoc-gui-test, and one used by the compiletest binary in `compiletest/src/bin/main.rs`.

All other modules (and crate-root items) are then made non-`pub` to achieve the API isolation. Doing so reveals some unused items, which have been removed.

(To reduce the amount of immediate textual churn, this PR does not comprehensively replace `pub` with `pub(crate)` throughout the whole crate; that could be done in a follow-up PR.)

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Ideally, rustdoc-gui-test would not depend on compiletest at all, but properly fixing that is out of scope for this PR.
- rust-lang/rust#143827

r? jieyouxu

Author: Zalathar