Bugfix: downloaded files may not match all environments

When Morgan mirrors packages, it chooses the most recent version that satisfies the requirement strings, and the environments. However, when matching environments, it would select files (and therefore version) that matches at least one of the Python versions of the environments, which could lead to Morgan choosing versions that do not support all of the Python versions.

This commit fixes this bug by requiring the selected versions match ALL of the Python versions defined in the environments. This could potentially lead to Morgan not finding matches at all, but this would be fixed in a subsequent release by treating each environment independently of the others.