Request: Stack Removal Migration Plan

[ekcasey]

Create an RFC describing migration steps and compatibility strategies/limitations for stack removal.

This should include migration paths and any gotchas for the following personas
a) buildpack author
b) stack author
c) platform author

It should also explicitly address:

1. any new API dependencies (e.g. does the lifecycle need to know about the distribution API?)
2. any potential limitations to our previous compatibility equation (any builder + any platform + any buildpack = success IFF the lifecycle supports the platform API and the buildpack API of all provided buildpacks).
3. a discussion of how compatibility will be maintained in the future given a change to any one of the specs. This may involve either:
   a) normalizing the specs to remove any potential for conflict
   b) assigning responsibility (e.g. to the platform or the lifecycle) for translating between specs to ensure compatibility
   c) explicitly accepting some amount of coupling and calling out any implications

This request is motivated by some specific tactical questions I added to the issue tracking stack removal implementation.