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

testing/test_pathlib.py

@@ -345,18 +345,18 @@ def test_resolve_package_path(tmp_path: Path) -> None:
     (pkg / "subdir").mkdir()
     (pkg / "subdir/__init__.py").touch()
     assert resolve_package_path(pkg) == pkg
-    assert resolve_package_path(pkg.joinpath("subdir", "__init__.py")) == pkg
+    assert resolve_package_path(pkg / "subdir/__init__.py") == pkg
 
 
 def test_package_unimportable(tmp_path: Path) -> None:
     pkg = tmp_path / "pkg1-1"
     pkg.mkdir()
     pkg.joinpath("__init__.py").touch()
-    subdir = pkg.joinpath("subdir")
+    subdir = pkg / "subdir"
     subdir.mkdir()
-    pkg.joinpath("subdir/__init__.py").touch()
+    (pkg / "subdir/__init__.py").touch()
     assert resolve_package_path(subdir) == subdir
-    xyz = subdir.joinpath("xyz.py")
+    xyz = subdir / "xyz.py"
     xyz.touch()
     assert resolve_package_path(xyz) == subdir
     assert not resolve_package_path(pkg)