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>

tests/unit/libutil/executable-path.cc

@@ -0,0 +1,64 @@
+#include <gtest/gtest.h>
+
+#include "executable-path.hh"
+
+namespace nix {
+
+#ifdef WIN32
+#  define PATH_VAR_SEP L";"
+#else
+#  define PATH_VAR_SEP ":"
+#endif
+
+#define PATH_ENV_ROUND_TRIP(NAME, STRING_LIT, CXX_LIT) \
+    TEST(ExecutablePath, NAME)                         \
+    {                                                  \
+        OsString s = STRING_LIT;                       \
+        auto v = ExecutablePath::parse(s);             \
+        EXPECT_EQ(v, (ExecutablePath CXX_LIT));        \
+        auto s2 = v.render();                          \
+        EXPECT_EQ(s2, s);                              \
+    }
+
+PATH_ENV_ROUND_TRIP(emptyRoundTrip, OS_STR(""), ({}))
+
+PATH_ENV_ROUND_TRIP(
+    oneElemRoundTrip,
+    OS_STR("/foo"),
+    ({
+        OS_STR("/foo"),
+    }))
+
+PATH_ENV_ROUND_TRIP(
+    twoElemsRoundTrip,
+    OS_STR("/foo" PATH_VAR_SEP "/bar"),
+    ({
+        OS_STR("/foo"),
+        OS_STR("/bar"),
+    }))
+
+PATH_ENV_ROUND_TRIP(
+    threeElemsRoundTrip,
+    OS_STR("/foo" PATH_VAR_SEP "." PATH_VAR_SEP "/bar"),
+    ({
+        OS_STR("/foo"),
+        OS_STR("."),
+        OS_STR("/bar"),
+    }))
+
+TEST(ExecutablePath, elementyElemNormalize)
+{
+    auto v = ExecutablePath::parse(PATH_VAR_SEP PATH_VAR_SEP PATH_VAR_SEP);
+    EXPECT_EQ(
+        v,
+        (ExecutablePath{{
+            OS_STR("."),
+            OS_STR("."),
+            OS_STR("."),
+            OS_STR("."),
+        }}));
+    auto s2 = v.render();
+    EXPECT_EQ(s2, OS_STR("." PATH_VAR_SEP "." PATH_VAR_SEP "." PATH_VAR_SEP "."));
+}
+
+}