compiletest: Isolate public APIs and minimize public surface area

[Zalathar]

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.

• src/tools/rustdoc-gui-test uses compiletest directives + TestProp handling internals in a questionable way #143827

r? jieyouxu