[module-loaders] Further simplify asset checks path (#26527)

## Summary & Motivation
We can further simplify the load_asset_from_x code path by making the
asset checks loader return AssetsDefinitions, and changing the
assertions to just make sure that the returned assets only contain
checks.

## How I Tested These Changes
Altered existing tests

python_modules/dagster/dagster/_core/definitions/assets.py

@@ -80,6 +80,7 @@
 from dagster._utils.warnings import ExperimentalWarning, disable_dagster_warnings
 
 if TYPE_CHECKING:
+    from dagster._core.definitions.asset_checks import AssetChecksDefinition
     from dagster._core.definitions.graph_definition import GraphDefinition
 
 ASSET_SUBSET_INPUT_PREFIX = "__subset_input__"
@@ -1180,6 +1181,30 @@ def get_op_def_for_asset_key(self, key: AssetKey) -> Optional[OpDefinition]:
         output_name = self.get_output_name_for_asset_key(key)
         return self.node_def.resolve_output_to_origin_op_def(output_name)
 
+    def coerce_to_checks_def(self) -> "AssetChecksDefinition":
+        from dagster._core.definitions.asset_checks import (
+            AssetChecksDefinition,
+            has_only_asset_checks,
+        )
+
+        if not has_only_asset_checks(self):
+            raise DagsterInvalidDefinitionError(
+                "Cannot coerce an AssetsDefinition to an AssetChecksDefinition if it contains "
+                "non-check assets."
+            )
+        if len(self.check_keys) == 0:
+            raise DagsterInvalidDefinitionError(
+                "Cannot coerce an AssetsDefinition to an AssetChecksDefinition if it contains no "
+                "checks."
+            )
+        return AssetChecksDefinition.create(
+            keys_by_input_name=self.keys_by_input_name,
+            node_def=self.op,
+            check_specs_by_output_name=self.check_specs_by_output_name,
+            resource_defs=self.resource_defs,
+            can_subset=self.can_subset,
+        )
+
     def with_attributes(
         self,
         *,

python_modules/dagster/dagster/_core/definitions/load_assets_from_modules.py

@@ -446,15 +446,11 @@ def replace_keys_in_asset(
                 key: key_replacements.get(key, key) for key in asset.keys_by_input_name.values()
             },
         )
-        if isinstance(asset, AssetsDefinition) and has_only_asset_checks(asset):
-            updated_object = AssetChecksDefinition.create(
-                keys_by_input_name=updated_object.keys_by_input_name,
-                node_def=updated_object.op,
-                check_specs_by_output_name=updated_object.check_specs_by_output_name,
-                resource_defs=updated_object.resource_defs,
-                can_subset=updated_object.can_subset,
-            )
-        return updated_object
+        return (
+            updated_object.coerce_to_checks_def()
+            if has_only_asset_checks(updated_object)
+            else updated_object
+        )
 
 
 class ResolvedAssetObjectList:
@@ -564,15 +560,11 @@ def with_attributes(
                 ).with_attributes(
                     backfill_policy=backfill_policy, freshness_policy=freshness_policy
                 )
-                if isinstance(asset, AssetChecksDefinition):
-                    new_asset = AssetChecksDefinition.create(
-                        keys_by_input_name=new_asset.keys_by_input_name,
-                        node_def=new_asset.op,
-                        check_specs_by_output_name=new_asset.check_specs_by_output_name,
-                        resource_defs=new_asset.resource_defs,
-                        can_subset=new_asset.can_subset,
-                    )
-                return_list.append(new_asset)
+                return_list.append(
+                    new_asset.coerce_to_checks_def()
+                    if has_only_asset_checks(new_asset)
+                    else new_asset
+                )
             elif isinstance(asset, SourceAsset):
                 return_list.append(
                     asset.with_attributes(group_name=group_name if group_name else asset.group_name)