fix(runfiles): correct Python runfiles path assumption

The current _FindPythonRunfilesRoot() implementation assumes that
the Python module has been unpacked four levels below the runfiles
directory. This is not the case in multiple situations, for example when
rules_pycross is in use and has installed the module via pypi (in which
case it is five levels below runfiles).

Both strategies already know where the runfiles directory exists -
implement _GetRunfilesDir() on the _DirectoryBased strategy, then call
_GetRunfilesDir() in order to populate self._python_runfiles_dir.

Fixes #3085

Author: jsing-canva

CHANGELOG.md

@@ -86,6 +86,10 @@ END_UNRELEASED_TEMPLATE
   ({gh-issue}`3043`).
 * (pypi) The pipstar `defaults` configuration now supports any custom platform
   name.
+* (runfiles) Fix incorrect Python runfiles path assumption - the existing
+  implementation assumes that it is always four levels below the runfiles
+  directory, leading to incorrect path checks
+  ([#3085](https://github.com/bazel-contrib/rules_python/issues/3085)).
 
 {#v0-0-0-added}
 ### Added

python/runfiles/runfiles.py

@@ -112,6 +112,9 @@ def RlocationChecked(self, path: str) -> str:
         # runfiles strategy on those platforms.
         return posixpath.join(self._runfiles_root, path)
 
+    def _GetRunfilesDir(self) -> str:
+        return self._runfiles_root
+
     def EnvVars(self) -> Dict[str, str]:
         return {
             "RUNFILES_DIR": self._runfiles_root,
@@ -129,7 +132,7 @@ class Runfiles:
 
     def __init__(self, strategy: Union[_ManifestBased, _DirectoryBased]) -> None:
         self._strategy = strategy
-        self._python_runfiles_root = _FindPythonRunfilesRoot()
+        self._python_runfiles_root = strategy._GetRunfilesDir()
         self._repo_mapping = _ParseRepoMapping(
             strategy.RlocationChecked("_repo_mapping")
         )
@@ -347,18 +350,6 @@ def Create(env: Optional[Dict[str, str]] = None) -> Optional["Runfiles"]:
 _Runfiles = Runfiles
 
 
-def _FindPythonRunfilesRoot() -> str:
-    """Finds the root of the Python runfiles tree."""
-    root = __file__
-    # Walk up our own runfiles path to the root of the runfiles tree from which
-    # the current file is being run. This path coincides with what the Bazel
-    # Python stub sets up as sys.path[0]. Since that entry can be changed at
-    # runtime, we rederive it here.
-    for _ in range("rules_python/python/runfiles/runfiles.py".count("/") + 1):
-        root = os.path.dirname(root)
-    return root
-
-
 def _ParseRepoMapping(repo_mapping_path: Optional[str]) -> Dict[Tuple[str, str], str]:
     """Parses the repository mapping manifest."""
     # If the repository mapping file can't be found, that is not an error: We