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>

.clang-format

@@ -31,3 +31,4 @@ AlwaysBreakBeforeMultilineStrings: true
 IndentPPDirectives: AfterHash
 PPIndentWidth: 2
 BinPackArguments: false
+BreakBeforeTernaryOperators: true