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.
It is considered bad form to use things from includes in headers, as
these directives propagate to everywhere else and can make it
confusing.
types.hh (which is includes almost literally everywhere) had some of
these directives, which this commit removes.
These were not caught by the previous clang-tidy invocation, but were
instead sorted out using amber[0] as such:
ambr --regex 'if (\(.+\))\s([a-z].*;)' 'if $1 { $2 }'
[0]: https://github.com/dalance/amber
