Separate headers from source files

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)

src/libutil/windows/environment-variables.cc

@@ -1,4 +1,4 @@
-#include "environment-variables.hh"
+#include "nix/environment-variables.hh"
 
 #ifdef _WIN32
 #  include "processenv.h"

src/libutil/windows/file-descriptor.cc

@@ -1,9 +1,9 @@
-#include "file-system.hh"
-#include "signals.hh"
-#include "finally.hh"
-#include "serialise.hh"
-#include "windows-error.hh"
-#include "file-path.hh"
+#include "nix/file-system.hh"
+#include "nix/signals.hh"
+#include "nix/finally.hh"
+#include "nix/serialise.hh"
+#include "nix/windows-error.hh"
+#include "nix/file-path.hh"
 
 #ifdef _WIN32
 #include <fileapi.h>

src/libutil/windows/file-path.cc

@@ -3,9 +3,9 @@
 #include <iostream>
 #include <locale>
 
-#include "file-path.hh"
-#include "file-path-impl.hh"
-#include "util.hh"
+#include "nix/file-path.hh"
+#include "nix/file-path-impl.hh"
+#include "nix/util.hh"
 
 namespace nix {
 

src/libutil/windows/file-system.cc

@@ -1,4 +1,4 @@
-#include "file-system.hh"
+#include "nix/file-system.hh"
 
 #ifdef _WIN32
 namespace nix {

src/libutil/windows/include/nix/meson.build

@@ -0,0 +1,9 @@
+# Public headers directory
+
+include_dirs += include_directories('..')
+
+headers += files(
+  'signals-impl.hh',
+  'windows-async-pipe.hh',
+  'windows-error.hh',
+)

src/libutil/windows/include/nix/signals-impl.hh

@@ -1,7 +1,7 @@
 #pragma once
 ///@file
 
-#include "types.hh"
+#include "nix/types.hh"
 
 namespace nix {
 

src/libutil/windows/include/nix/windows-async-pipe.hh

@@ -1,7 +1,7 @@
 #pragma once
 ///@file
 
-#include "file-descriptor.hh"
+#include "nix/file-descriptor.hh"
 #ifdef _WIN32
 
 namespace nix::windows {

src/libutil/windows/include/nix/windows-error.hh

@@ -4,7 +4,7 @@
 #ifdef _WIN32
 #include <errhandlingapi.h>
 
-#include "error.hh"
+#include "nix/error.hh"
 
 namespace nix::windows {
 

src/libutil/windows/meson.build

@@ -11,10 +11,4 @@ sources += files(
   'windows-error.cc',
 )
 
-include_dirs += include_directories('.')
-
-headers += files(
-  'signals-impl.hh',
-  'windows-async-pipe.hh',
-  'windows-error.hh',
-)
+subdir('include/nix')

src/libutil/windows/muxable-pipe.cc

@@ -1,10 +1,10 @@
 #ifdef _WIN32
 #  include <ioapiset.h>
-#  include "windows-error.hh"
+#  include "nix/windows-error.hh"
 
-#  include "logging.hh"
-#  include "util.hh"
-#  include "muxable-pipe.hh"
+#  include "nix/logging.hh"
+#  include "nix/util.hh"
+#  include "nix/muxable-pipe.hh"
 
 namespace nix {
 

src/libutil/windows/os-string.cc

@@ -3,9 +3,9 @@
 #include <iostream>
 #include <locale>
 
-#include "file-path.hh"
-#include "file-path-impl.hh"
-#include "util.hh"
+#include "nix/file-path.hh"
+#include "nix/file-path-impl.hh"
+#include "nix/util.hh"
 
 #ifdef _WIN32
 

src/libutil/windows/processes.cc

@@ -1,16 +1,16 @@
-#include "current-process.hh"
-#include "environment-variables.hh"
-#include "error.hh"
-#include "executable-path.hh"
-#include "file-descriptor.hh"
-#include "file-path.hh"
-#include "signals.hh"
-#include "processes.hh"
-#include "finally.hh"
-#include "serialise.hh"
-#include "file-system.hh"
-#include "util.hh"
-#include "windows-error.hh"
+#include "nix/current-process.hh"
+#include "nix/environment-variables.hh"
+#include "nix/error.hh"
+#include "nix/executable-path.hh"
+#include "nix/file-descriptor.hh"
+#include "nix/file-path.hh"
+#include "nix/signals.hh"
+#include "nix/processes.hh"
+#include "nix/finally.hh"
+#include "nix/serialise.hh"
+#include "nix/file-system.hh"
+#include "nix/util.hh"
+#include "nix/windows-error.hh"
 
 #include <cerrno>
 #include <cstdlib>

src/libutil/windows/users.cc

@@ -1,8 +1,8 @@
-#include "util.hh"
-#include "users.hh"
-#include "environment-variables.hh"
-#include "file-system.hh"
-#include "windows-error.hh"
+#include "nix/util.hh"
+#include "nix/users.hh"
+#include "nix/environment-variables.hh"
+#include "nix/file-system.hh"
+#include "nix/windows-error.hh"
 
 #ifdef _WIN32
 #define WIN32_LEAN_AND_MEAN

src/libutil/windows/windows-async-pipe.cc

@@ -1,5 +1,5 @@
-#include "windows-async-pipe.hh"
-#include "windows-error.hh"
+#include "nix/windows-async-pipe.hh"
+#include "nix/windows-error.hh"
 
 #ifdef _WIN32
 

src/libutil/windows/windows-error.cc

@@ -1,4 +1,4 @@
-#include "windows-error.hh"
+#include "nix/windows-error.hh"
 
 #ifdef _WIN32
 #include <error.h>