Fix SPDX headers in all files

I ran the `bin/console spdx --fix` with different strategies for
different files. For most of the core classes, since they've been
drastically rebuilt, I've run it with the `git-blame` strategy, for for
the `src/Validators`, in which the API changed completely but the logic
remains the same, I use the `git-log` strategy.

Author: henriquemoody