This also required fixing many, many, many problems with the existing
binary-dist and test rules, so as to be able to indifferently test an
in-tree compiler or one that comes from a binary distribution, both on my
Linux and Windows systems.

We can't quite run './validate --hadrian' to completion just yet on Windows, it
fails right before running the tests on my Windows system, see #16353.
It does work on Linux and returns the exact same results as when
testing the in-tree compiler, modulo the few tests that we only run for an
in-tree compiler.

To make binary distributions work on my local systems and in CI, I've had to
disable the generation of Haddock's PDF manual with LaTeX. This is tracked
at #16352.