pipeline: drop output_id and pull in output-directory

Now that no caller requires the "output_id" anymore, drop it from our
results-dictionary. Instead, pass the output-directory through and copy
outputs where we produce / fetch them.

This still uses `objectstore.resolve_ref()`, since we do not have the
outputs pinned at the places where we want to copy. This needs a little
bit more rework, but we might just delay that until we have the cache
rework landed.

This already simplifies the output-directory path and drops the slight
hack which checked very late for produced outputs.

Note that we must be careful not to copy things too early, because we
do not want remnants in the output-directory if we return failure.
Hence, keep the copy-operation close to the commit-operation on the
store.

osbuild/objectstore.py

@@ -195,6 +195,20 @@ class Object:
     def __exit__(self, exc_type, exc_val, exc_tb):
         self.cleanup()
 
+    def export(self, to_directory):
+        """Copy object into an external directory"""
+        with self.read() as from_directory:
+            subprocess.run(
+                [
+                    "cp",
+                    "--reflink=auto",
+                    "-a",
+                    f"{from_directory}/.",
+                    to_directory,
+                ],
+                check=True,
+            )
+
 
 class HostTree:
     """Read-only access to the host file system