Add Composer support to PMPro plugin(s)

[jackvargo]

Is your feature request related to a problem? Please describe.
We are converting to a modern Wordpress development process and need the PMPro plugins to support Composer-based workflows. I am trying to avoid adding references to each VCS path in my composer configuration, and will request the packages be submitted to Packagist, or will offer to help with that if the PR is accepted.

Describe the solution you'd like
Add basic Composer support.

Describe alternatives you've considered
I opened a support ticket and was advised this would be considered by the developers, and suggested to fork the plugins, add support, and submit a PR to incorporate into the core plugins.

Additional context
N/a.