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 reverts commit 2ab66390f8a461b074cb3cf6073b8f9d60875436.

Fixes a crash that has been present on Android (and probably also
macOS) since f6883a0.

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.

Now that we utilize C++17, we can simply return an optional containing
the code instead of using an out variable and a boolean result,
essentially combining them into one.

This provides a much more straightforward interface.