Debug output control for import-source-packages.py

[Bryce Harrington]

The import-source-packages.py script is a bit chatty with debug information. Now that we capture the output in systemd's journal, this is adding noise to the logs.

The output may be useful for debugging purposes but I think should be suppressed for normal operation.

I'm imagining replacing print() calls in import-source-packages.py with calls to info()/warn()/die() routines, which use either a DEBUG env var or a --verbose/--debug type command line argument (or both) to control whether to show output.

Some of the types of messages I notice, that could be filtered out:

Nov 01 16:44:14 git-ubuntu git-ubuntu.import-source-packages[30185]: 'openvswitch'}
Nov 01 16:44:14 git-ubuntu git-ubuntu.import-source-packages[30185]: 'maven-repo-helper',
Nov 01 16:44:14 git-ubuntu git-ubuntu.import-source-packages[30185]: 'libvoikko',
Nov 01 16:44:14 git-ubuntu git-ubuntu.import-source-packages[30185]: 'libtommath',
Nov 01 16:44:14 git-ubuntu git-ubuntu.import-source-packages[30185]: 'libspiro',
Nov 01 16:44:14 git-ubuntu git-ubuntu.import-source-packages[30185]: {'libshout',
Nov 01 16:44:14 git-ubuntu git-ubuntu.import-source-packages[30185]: Requested imports for:

Nov 01 16:01:28 git-ubuntu git-ubuntu.import-source-packages[30185]: Imported 0 source packages
Nov 01 16:24:10 git-ubuntu git-ubuntu.import-source-packages[30185]: set()
Nov 01 16:24:10 git-ubuntu git-ubuntu.import-source-packages[30185]: Requested imports for:

Nov 01 12:28:56 git-ubuntu git-ubuntu.import-source-packages[30185]: No new relevant publishes found in debian relative to 2019-11-01 10:37:
Nov 01 12:28:54 git-ubuntu git-ubuntu.import-source-packages[30185]: Examining publishes in debian since 2019-11-01 10:37:01