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/meson.build

@@ -129,6 +129,7 @@ sources = files(
   'english.cc',
   'environment-variables.cc',
   'error.cc',
+  'executable-path.cc',
   'exit.cc',
   'experimental-features.cc',
   'file-content-address.cc',
@@ -183,6 +184,7 @@ headers = [config_h] + files(
   'english.hh',
   'environment-variables.hh',
   'error.hh',
+  'executable-path.hh',
   'exit.hh',
   'experimental-features.hh',
   'file-content-address.hh',
@@ -202,6 +204,7 @@ headers = [config_h] + files(
   'lru-cache.hh',
   'memory-source-accessor.hh',
   'muxable-pipe.hh',
+  'os-string.hh',
   'pool.hh',
   'position.hh',
   'posix-source-accessor.hh',