Bugfix: file parsing failures could be detrimental

This commit fixes two issues - both related to archive parsing failures

• that could be fatal to the mirroring process or otherwise detrimental
  with the process:

1. On Python versions prior to 3.10, failure to parse an archive could
   result in the process exiting instead of continuing with the next
   requirement.

2. Failure to parse metadata file could result in the metadata file not
   being stored in the index.

Related code is also refactored for better maintenance and readability,
and the task of writing an archive's main METADATA file into the index
is relegated to the morgan.metadata module.