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/strings.cc

@@ -1,4 +1,5 @@
 #include <gtest/gtest.h>
+#include <rapidcheck/gtest.h>
 
 #include "strings.hh"
 
@@ -231,4 +232,117 @@ TEST(tokenizeString, tokenizeSepEmpty)
     ASSERT_EQ(tokenizeString<Strings>(s, ","), expected);
 }
 
+/* ----------------------------------------------------------------------------
+ * splitString
+ * --------------------------------------------------------------------------*/
+
+TEST(splitString, empty)
+{
+    Strings expected = {""};
+
+    ASSERT_EQ(splitString<Strings>("", " \t\n\r"), expected);
+}
+
+TEST(splitString, oneSep)
+{
+    Strings expected = {"", ""};
+
+    ASSERT_EQ(splitString<Strings>(" ", " \t\n\r"), expected);
+}
+
+TEST(splitString, twoSep)
+{
+    Strings expected = {"", "", ""};
+
+    ASSERT_EQ(splitString<Strings>(" \n", " \t\n\r"), expected);
+}
+
+TEST(splitString, tokenizeSpacesWithSpaces)
+{
+    auto s = "foo bar baz";
+    Strings expected = {"foo", "bar", "baz"};
+
+    ASSERT_EQ(splitString<Strings>(s, " \t\n\r"), expected);
+}
+
+TEST(splitString, tokenizeTabsWithDefaults)
+{
+    auto s = "foo\tbar\tbaz";
+    // Using it like this is weird, but shows the difference with tokenizeString, which also has this test
+    Strings expected = {"foo", "bar", "baz"};
+
+    ASSERT_EQ(splitString<Strings>(s, " \t\n\r"), expected);
+}
+
+TEST(splitString, tokenizeTabsSpacesWithDefaults)
+{
+    auto s = "foo\t bar\t baz";
+    // Using it like this is weird, but shows the difference with tokenizeString, which also has this test
+    Strings expected = {"foo", "", "bar", "", "baz"};
+
+    ASSERT_EQ(splitString<Strings>(s, " \t\n\r"), expected);
+}
+
+TEST(splitString, tokenizeTabsSpacesNewlineWithDefaults)
+{
+    auto s = "foo\t\n bar\t\n baz";
+    // Using it like this is weird, but shows the difference with tokenizeString, which also has this test
+    Strings expected = {"foo", "", "", "bar", "", "", "baz"};
+
+    ASSERT_EQ(splitString<Strings>(s, " \t\n\r"), expected);
+}
+
+TEST(splitString, tokenizeTabsSpacesNewlineRetWithDefaults)
+{
+    auto s = "foo\t\n\r bar\t\n\r baz";
+    // Using it like this is weird, but shows the difference with tokenizeString, which also has this test
+    Strings expected = {"foo", "", "", "", "bar", "", "", "", "baz"};
+
+    ASSERT_EQ(splitString<Strings>(s, " \t\n\r"), expected);
+
+    auto s2 = "foo \t\n\r bar \t\n\r baz";
+    Strings expected2 = {"foo", "", "", "", "", "bar", "", "", "", "", "baz"};
+
+    ASSERT_EQ(splitString<Strings>(s2, " \t\n\r"), expected2);
+}
+
+TEST(splitString, tokenizeWithCustomSep)
+{
+    auto s = "foo\n,bar\n,baz\n";
+    Strings expected = {"foo\n", "bar\n", "baz\n"};
+
+    ASSERT_EQ(splitString<Strings>(s, ","), expected);
+}
+
+TEST(splitString, tokenizeSepAtStart)
+{
+    auto s = ",foo,bar,baz";
+    Strings expected = {"", "foo", "bar", "baz"};
+
+    ASSERT_EQ(splitString<Strings>(s, ","), expected);
+}
+
+TEST(splitString, tokenizeSepAtEnd)
+{
+    auto s = "foo,bar,baz,";
+    Strings expected = {"foo", "bar", "baz", ""};
+
+    ASSERT_EQ(splitString<Strings>(s, ","), expected);
+}
+
+TEST(splitString, tokenizeSepEmpty)
+{
+    auto s = "foo,,baz";
+    Strings expected = {"foo", "", "baz"};
+
+    ASSERT_EQ(splitString<Strings>(s, ","), expected);
+}
+
+// concatStringsSep sep . splitString sep = id   if sep is 1 char
+RC_GTEST_PROP(splitString, recoveredByConcatStringsSep, (const std::string & s))
+{
+    RC_ASSERT(concatStringsSep("/", splitString<Strings>(s, "/")) == s);
+    RC_ASSERT(concatStringsSep("a", splitString<Strings>(s, "a")) == s);
+}
+
 } // namespace nix