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/meson.build

@@ -52,6 +52,7 @@ sources = files(
   'closure.cc',
   'compression.cc',
   'config.cc',
+  'executable-path.cc',
   'file-content-address.cc',
   'git.cc',
   'hash.cc',