Fix reading number of dags from direct system providers path (apache#50843)

The original PR apache#49978 that fixed generation of proper documentation for
standard provider examples added retrieval of the path by checking
import location for the standard provider. Instead we are now using
direct path to the provider.

Author: potiuk

airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_dag_report.py

@@ -23,7 +23,7 @@
 
 from airflow.utils.file import list_py_file_paths
 
-import system.standard
+from tests_common.pytest_plugin import AIRFLOW_ROOT_PATH
 from tests_common.test_utils.config import conf_vars
 from tests_common.test_utils.db import clear_db_dags, parse_and_sync_to_db
 
@@ -34,14 +34,18 @@
 TEST_DAG_FOLDER_INVALID = "/invalid/path"
 TEST_DAG_FOLDER_INVALID_2 = "/root/airflow/tests/dags/"
 
+STANDARD_PROVIDER_SYSTEM_TESTS_PATH = (
+    AIRFLOW_ROOT_PATH / "providers" / "standard" / "tests" / "system" / "standard"
+)
+
 
 def get_corresponding_dag_file_count(dir: str, include_examples: bool = True) -> int:
     from airflow import example_dags
 
     return (
         len(list_py_file_paths(directory=dir))
         + (len(list_py_file_paths(next(iter(example_dags.__path__)))) if include_examples else 0)
-        + (len(list_py_file_paths(next(iter(system.standard.__path__)))) if include_examples else 0)
+        + (len(list_py_file_paths(STANDARD_PROVIDER_SYSTEM_TESTS_PATH.as_posix())) if include_examples else 0)
     )