It doesn't serve much of a purpose since we can easily put its contents
inside __config. Removing it simplifies the modulemap once we are trying
to create a single top-level module.
I don't know when, but at some point we lost test coverage to ensue that
all the headers are in the modulemap. This adds a test to make sure all
the headers (excluding a few which shouldn't be part of the modulemap)
are at least mentioned. This also fixes a few headers which bit-rotted
while we were missing the coverage.
