Refactor: use division operator to join paths (#11413)

Starting with `resolve_package_path` and its associated tests,
this refactoring seeks to make path concatenation more
readable and consistent within tests/functions.

As discussed in #11413:

- code is free to use either `/` and `joinpath`
- consistency within a function is more important than consistency across the codebase
- it is nice to use `/` when it is more readable
- it is nice to use `joinpath` when there is little context
- be mindful that `joinpath` may be clearer when joining multiple segments

src/_pytest/pathlib.py

@@ -680,7 +680,7 @@ def resolve_package_path(path: Path) -> Optional[Path]:
     result = None
     for parent in itertools.chain((path,), path.parents):
         if parent.is_dir():
-            if not parent.joinpath("__init__.py").is_file():
+            if not (parent / "__init__.py").is_file():
                 break
             if not parent.name.isidentifier():
                 break