Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

This fixes a bunch of DEBUG_ASSERTs in the unit tests.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

We use more or less the same values for all PowerPC float unit
tests. Let's put them in one place instead of duplicating them.

If the compiler can detect an issue with a format string at compile
time, then we should take advantage of that and turn the issue into a
hard compile-time error as such problems almost always lead to UB.

This helps with catching logging or assertion messages that have been
converted over to fmt but are still using the old, non-fmt variants
of the logging macros.

This commit also fixes all incorrect usages that I could find.