The short answer for why we need to do this is so we can consistently do
`#include "nix/..."`. Without this change, there are ways to still make
that work, but they are hacky, and they have downsides such as making it
harder to make sure headers from the wrong Nix library (e..g.
`libnixexpr` headers in `libnixutil`) aren't being used.
The C API alraedy used `nix_api_*`, so its headers are *not* put in
subdirectories accordingly.
Progress on #7876
We resisted doing this for a while because it would be annoying to not
have the header source file pairs close by / easy to change file
path/name from one to the other. But I am ameliorating that with
symlinks in the next commit.
(cherry picked from commit f3e1c47f47)
Unfortunately `StringSource` class is very easy was very easy to misuse
because the ctor took a plain `std::string_view` which has a bad habit
of being implicitly convertible from an rvalue `std::string`. This lead
to unintentional use-after-free bugs.
This patch makes `StringSource` much harder to misuse by disabling the ctor
from a `std::string &&` (but `const std::string &` is ok).
Fix affected tests from libstore-tests.
Reformat those tests with clangd's range formatting since the diff is tiny
and it seems appropriate.