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.hh

@@ -9,6 +9,7 @@
 #include <optional>
 
 #include "types.hh"
+#include "file-path.hh"
 
 namespace nix {
 
@@ -17,6 +18,11 @@ namespace nix {
  */
 std::optional<std::string> getEnv(const std::string & key);
 
+/**
+ * Like `getEnv`, but using `OsString` to avoid coercions.
+ */
+std::optional<OsString> getEnvOs(const OsString & key);
+
 /**
  * @return a non empty environment variable. Returns nullopt if the env
  * variable is set to ""
@@ -43,6 +49,11 @@ int unsetenv(const char * name);
  */
 int setEnv(const char * name, const char * value);
 
+/**
+ * Like `setEnv`, but using `OsString` to avoid coercions.
+ */
+int setEnvOs(const OsString & name, const OsString & value);
+
 /**
  * Clear the environment.
  */