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.
Reformatted with: fd . -e hh -e cc | xargs clang-format -i
git-subtree-dir: third_party/nix git-subtree-mainline: cf8cd640c1 git-subtree-split: be66c7a6b24e3c3c6157fd37b86c7203d14acf10
cf8cd640c1