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.
Avoids propagating headers into scopes where they're not necessary.
Also uncovered reliance on an indirect inclusion within
CheatsManager.cpp, which is now fixed.
