Experimental Package Manager Plugin Framework

[adambkaplan]

Following up on a live conversation between @eskultety, @brunoapimentel, and I had, Hermeto should provide a plugin framework that allows experimental package managers to be iterated on. This was sparked by the challenges with Maven and its ability to provide the necessary inputs to Hermeto (see #1246).

Such a plugin framework would need to do the following:

• Provide a clear interface that package managers need to implement.
• Provide mechanisms for package manager plugins to be included as part of a Hermeto "distribution."
• Establish verification patterns and practices (ex: unit and integration testing via CI)
• Define criteria and process for package managers to "graduate" to Hermeto's core codebase.