pipeline, fmt: use build result object internally

Instead of serializing the `BuildResult` to a dict in `build_stages`,
we keep the object and then only serialize it in the corresponding
formatting code. This doubles down on the separation between the
internal data structures and the external representation of them. It
was partially already done in the v2 format which hand-picked which
elements of the BuildResult it would return for each stage.

osbuild/formats/v1.py

@@ -9,7 +9,7 @@ to fetch resources.
 """
 from typing import Dict
 from osbuild.meta import Index, ValidationResult
-from ..pipeline import Manifest, Pipeline, detect_host_runner
+from ..pipeline import BuildResult, Manifest, Pipeline, detect_host_runner
 
 
 VERSION = "1"
@@ -194,6 +194,16 @@ def load(description: Dict, index: Index) -> Manifest:
 def output(manifest: Manifest, res: Dict) -> Dict:
     """Convert a result into the v1 format"""
 
+    def result_for_stage(result: BuildResult):
+        return {
+            "id": result.id,
+            "type": result.name,
+            "success": result.success,
+            "error": result.error,
+            "output": result.output,
+            "metadata": result.metadata,
+        }
+
     def result_for_pipeline(pipeline):
         # The pipeline might not have been built one of its
         # dependencies, i.e. its build pipeline, failed to
@@ -211,7 +221,9 @@ def output(manifest: Manifest, res: Dict) -> Dict:
 
         stages = current.get("stages")
         if stages:
-            retval["stages"] = stages
+            retval["stages"] = [
+                result_for_stage(r) for r in stages
+            ]
         return retval
 
     result = result_for_pipeline(manifest["tree"])
@@ -226,7 +238,10 @@ def output(manifest: Manifest, res: Dict) -> Dict:
     if not current:
         return result
 
-    result["assembler"] = current["stages"][0]
+    # The assembler pipeline must have exactly one stage
+    # which is the v1 assembler
+    stage = current["stages"][0]
+    result["assembler"] = result_for_stage(stage)
     if not result["assembler"]["success"]:
         result["success"] = False