don't allow loading stages without cmd

This was loosened when introducing parametrization.
But, `foreach`..`do` should be considered first
before stage's regular structure. And, cmd is made
required (as it should have been)

Related: iterative#5371, iterative#5370, iterative#5312

dvc/schema.py

@@ -1,4 +1,4 @@
-from voluptuous import Any, Optional, Required, Schema
+from voluptuous import Any, Exclusive, Optional, Required, Schema
 
 from dvc import dependency, output
 from dvc.hash_info import HashInfo
@@ -79,7 +79,7 @@
 VARS_SCHEMA = [str, dict]
 
 STAGE_DEFINITION = {
-    StageParams.PARAM_CMD: Any(str, list),
+    Required(StageParams.PARAM_CMD): Any(str, list),
     Optional(StageParams.PARAM_WDIR): str,
     Optional(StageParams.PARAM_DEPS): [str],
     Optional(StageParams.PARAM_PARAMS): [Any(str, dict)],
@@ -100,7 +100,7 @@
     Required(FOREACH_KWD): Any(dict, list, str),
     Required(DO_KWD): STAGE_DEFINITION,
 }
-SINGLE_PIPELINE_STAGE_SCHEMA = {str: Any(STAGE_DEFINITION, FOREACH_IN)}
+SINGLE_PIPELINE_STAGE_SCHEMA = {str: Exclusive(FOREACH_IN, STAGE_DEFINITION)}
 MULTI_STAGE_SCHEMA = {
     STAGES: SINGLE_PIPELINE_STAGE_SCHEMA,
     VARS_KWD: VARS_SCHEMA,