Extra input validation for multiple executors (apache#39067)

Ensure the user provides the module path as the second portion of the
alias:module_path input, not the other way around

airflow/executors/executor_loader.py

@@ -120,10 +120,11 @@ def _get_executor_names(cls) -> list[ExecutorName]:
                 # complicated. Multiple Executors of the same type will be supported by a future multitenancy
                 # AIP.
                 # The module component should always be a module or plugin path.
-                if not split_name[1] or split_name[1] in CORE_EXECUTOR_NAMES:
+                module_path = split_name[1]
+                if not module_path or module_path in CORE_EXECUTOR_NAMES or "." not in module_path:
                     raise AirflowConfigException(
-                        f"Incorrectly formatted executor configuration: {name}\n"
-                        "second portion of an executor configuration must be a module path"
+                        "Incorrectly formatted executor configuration. Second portion of an executor "
+                        f"configuration must be a module path or plugin but received: {module_path}"
                     )
                 else:
                     executor_names.append(ExecutorName(alias=split_name[0], module_path=split_name[1]))

tests/executors/test_executor_loader.py

@@ -205,6 +205,7 @@ def test_get_hybrid_executors_from_config_duplicates_should_fail(self, executor_
             "LocalExecutor, CeleryExecutor:, DebugExecutor",
             "LocalExecutor, my_cool_alias:",
             "LocalExecutor, my_cool_alias:CeleryExecutor",
+            "LocalExecutor, module.path.first:alias_second",
         ],
     )
     def test_get_hybrid_executors_from_config_core_executors_bad_config_format(self, executor_config):