Previously all includes were anchored in one global mess of header
files. This moves the includes into filesystem "namespaces" (if you
will) for each sub-package of Nix.
Note: This commit does not introduce the relevant build system changes.
Externally developed dependencies which have been imported ("vendored") into
this repository. These dependencies come with their own licenses and whatever
else.
Code that is developed inside of this repository, but released to an external
repository via Copybara.