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/environment-variables.cc

@@ -1,20 +1,23 @@
 #include "util.hh"
 #include "environment-variables.hh"
 
-extern char * * environ __attribute__((weak));
+extern char ** environ __attribute__((weak));
 
 namespace nix {
 
 std::optional<std::string> getEnv(const std::string & key)
 {
     char * value = getenv(key.c_str());
-    if (!value) return {};
+    if (!value)
+        return {};
     return std::string(value);
 }
 
-std::optional<std::string> getEnvNonEmpty(const std::string & key) {
+std::optional<std::string> getEnvNonEmpty(const std::string & key)
+{
     auto value = getEnv(key);
-    if (value == "") return {};
+    if (value == "")
+        return {};
     return value;
 }