Bugfix: some setuptools dependencies may be missed

Some dependencies define in a setuptools' requirements.txt file can be
missed and not mirrored if their are conditional on both an extra and
environment marker specifications.

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.

Improved and Independent Dependency Resolution

This release implements independent dependency resolution for Morgan. Previous releases used pkginfo, which misses a lot of dependencies, as now described in the project's README file. This means Morgan is now capable to better resolve dependencies, and also ensure that build dependencies are included in the mirror (e.g. setuptools, hatch, etc.).

Generate Requirements from Current Environment

This release adds the ability to generate a list of requirements for Morgan's
configuration file from the current environment, i.e. all packages that are
installed in sys.path. This functionality is provided via the generate_reqs
command, in a similar manner to generate_env.

Thanks to Nikolai Gaiduchenko for this new functionality.

First Public Release: v0.9.0

First public release.