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/executable-path.cc

@@ -0,0 +1,79 @@
+#include "environment-variables.hh"
+#include "executable-path.hh"
+#include "strings-inline.hh"
+#include "util.hh"
+#include "file-path-impl.hh"
+
+namespace nix {
+
+namespace fs = std::filesystem;
+
+constexpr static const OsStringView path_var_separator{
+    &ExecutablePath::separator,
+    1,
+};
+
+ExecutablePath ExecutablePath::load()
+{
+    // "If PATH is unset or is set to null, the path search is
+    // implementation-defined."
+    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
+    return ExecutablePath::parse(getEnvOs(OS_STR("PATH")).value_or(OS_STR("")));
+}
+
+ExecutablePath ExecutablePath::parse(const OsString & path)
+{
+    auto strings = path.empty() ? (std::list<OsString>{})
+                                : basicSplitString<std::list<OsString>, OsString::value_type>(path, path_var_separator);
+
+    std::vector<fs::path> ret;
+    ret.reserve(strings.size());
+
+    std::transform(
+        std::make_move_iterator(strings.begin()),
+        std::make_move_iterator(strings.end()),
+        std::back_inserter(ret),
+        [](auto && str) {
+            return fs::path{
+                str.empty()
+                    // "A zero-length prefix is a legacy feature that
+                    // indicates the current working directory. It
+                    // appears as two adjacent <colon> characters
+                    // ("::"), as an initial <colon> preceding the rest
+                    // of the list, or as a trailing <colon> following
+                    // the rest of the list."
+                    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
+                    ? OS_STR(".")
+                    : std::move(str),
+            };
+        });
+
+    return {ret};
+}
+
+OsString ExecutablePath::render() const
+{
+    std::vector<PathViewNG> path2;
+    for (auto & p : directories)
+        path2.push_back(p.native());
+    return basicConcatStringsSep(path_var_separator, path2);
+}
+
+std::optional<fs::path>
+ExecutablePath::find(const OsString & exe, std::function<bool(const fs::path &)> isExecutable) const
+{
+    // "If the pathname being sought contains a <slash>, the search
+    // through the path prefixes shall not be performed."
+    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
+    assert(OsPathTrait<fs::path::value_type>::rfindPathSep(exe) == exe.npos);
+
+    for (auto & dir : directories) {
+        auto candidate = dir / exe;
+        if (isExecutable(candidate))
+            return std::filesystem::canonical(candidate);
+    }
+
+    return std::nullopt;
+}
+
+} // namespace nix