`make check` was reverted too soon. The hacking guide wasn't brought
up to date with the new workflow, and it's not clear how to use
meson for everything.
This reverts commit 6f3045c2a2.
Now that we can run all tests with Meson, we want developers making code
changes to use it.
(Only the manual needs to be built with the build system, and that will
change shortly.)
This reverts commit b0bc2a97bf.
Progress on #5638
There are still a global fetcher and eval settings, but they are pushed
down into `libnixcmd`, which is a lot less bad a place for this sort of
thing.
Continuing process pioneered in
52bfccf8d8.
