chromium: Rename sources.nix to upstream-info.nix

The "sources.nix" also contains information about where to get binary
packages, so calling it "upstream-info.nix" fits better in terms of
naming.

Also, we're moving it away from the sources dir, because the latter will
soon vanish.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>