formats: extract manifest loading into module

Extract the code that loads a pipeline from a pipeline description,
i.e. a manifest, into a new module inside a new 'formats' package.
The idea is to have different descriptions, i.e. different formats,
for the same internal representation. This allows changing the
internal representation, i.e. data structures, but still having the
same external description.
Later a new description might be added that better matches the new
internal representation.

osbuild/formats/v1.py

@@ -0,0 +1,32 @@
+# Version 1 of the manifest description
+
+from ..pipeline import Pipeline, detect_host_runner
+
+
+def load_build(description, sources_options):
+    pipeline = description.get("pipeline")
+    if pipeline:
+        build_pipeline = load(pipeline, sources_options)
+    else:
+        build_pipeline = None
+
+    return build_pipeline, description["runner"]
+
+
+def load(description, sources_options):
+    build = description.get("build")
+    if build:
+        build_pipeline, runner = load_build(build, sources_options)
+    else:
+        build_pipeline, runner = None, detect_host_runner()
+
+    pipeline = Pipeline(runner, build_pipeline)
+
+    for s in description.get("stages", []):
+        pipeline.add_stage(s["name"], sources_options, s.get("options", {}))
+
+    a = description.get("assembler")
+    if a:
+        pipeline.set_assembler(a["name"], a.get("options", {}))
+
+    return pipeline