Merge branch 'chromium-source-refactor'

Fixes #12794 by reverting the source tree splitup (c92dbffe) to use the
source tarball directly into the main Chromium derivation and making the
whole source/ subdirectory obsolete. The reasons for this are explained
in 4f981b4f.

This also now renames the "sources.nix" file to "upstream-info.nix",
which is a more proper name for the file, because it not only contains
"source code" but also the Chrome binaries needed for the proprietary
plugins (of course "source" could also mean "where to get it", but I
wanted to avoid this ambiguity entirely).

I have successfully built and tested this using the VM tests.

All results can be found here:

https://headcounter.org/hydra/eval/313435