Factor out lookupExecutable and other PATH improvments

This ended up motivating a good deal of other infra improvements in
order to get Windows right:

- `OsString` to complement `std::filesystem::path`

- env var code for working with the underlying `OsString`s

- Rename `PATHNG_LITERAL` to `OS_STR`

- `NativePathTrait` renamed to `OsPathTrait`, given a character template
  parameter until #9205 is complete.

Split `tests.cc` matching split of `util.{cc,hh}` last year.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libutil/unix/environment-variables.cc

@@ -9,4 +9,14 @@ int setEnv(const char * name, const char * value)
     return ::setenv(name, value, 1);
 }
 
+std::optional<std::string> getEnvOs(const std::string & key)
+{
+    return getEnv(key);
+}
+
+int setEnvOs(const OsString & name, const OsString & value)
+{
+    return setEnv(name.c_str(), value.c_str());
+}
+
 }

src/libutil/unix/file-path.cc

@@ -8,16 +8,6 @@
 
 namespace nix {
 
-std::string os_string_to_string(PathViewNG::string_view path)
-{
-    return std::string { path };
-}
-
-std::filesystem::path::string_type string_to_os_string(std::string_view s)
-{
-    return std::string { s };
-}
-
 std::optional<std::filesystem::path> maybePath(PathView path)
 {
     return { path };

src/libutil/unix/meson.build

@@ -4,6 +4,7 @@ sources += files(
   'file-path.cc',
   'file-system.cc',
   'muxable-pipe.cc',
+  'os-string.cc',
   'processes.cc',
   'signals.cc',
   'users.cc',

src/libutil/unix/os-string.cc

@@ -0,0 +1,21 @@
+#include <algorithm>
+#include <codecvt>
+#include <iostream>
+#include <locale>
+
+#include "file-path.hh"
+#include "util.hh"
+
+namespace nix {
+
+std::string os_string_to_string(PathViewNG::string_view path)
+{
+    return std::string{path};
+}
+
+std::filesystem::path::string_type string_to_os_string(std::string_view s)
+{
+    return std::string{s};
+}
+
+}