osbuild: add workaround to integrate sources into progress reporting

This commit is somewhat poor, sorry for that. It mostly adds
workaround so that the osbuild sources can emit some progress
reporting as well. Without that the user experience is rather poor
and there is a long delay before any sort of progress can be
reported (even before the normal stages run).

With it the user experience is still not good but slightly better,
i.e. the progress monitor will report that the sources have
started downloading and curl will generated some log output. No
real progress unfortunately (sources subprogress will jump from
zero to 100%).

sources/org.osbuild.curl

@@ -192,6 +192,13 @@ class CurlSource(sources.SourceService):
                 os.rename(f"{tmpdir}/{checksum}", f"{self.cache}/{checksum}")
             except FileExistsError:
                 pass
+            # Workaround the lack of structured progress reporting from
+            # stages/sources. It generates messages of the form
+            #   "message": "source/org.osbuild.curl (org.osbuild.curl): Downloaded https://rpmrepo.osbuild.org/v2/mirror/public/f38/f38-x86_64-fedora-20230413/Packages/f/fonts-srpm-macros-2.0.5-11.fc38.noarch.rpm\n
+            #
+            # Without it just a long pause with no progress while curl
+            # downloads.
+            print(f"Downloaded {url}")
 
 
 def main():