[RFC 0069] Support Stack Buildpacks

[ekcasey]

RFC 0069
buildpacks/rfcs#111

buildpacks/spec#177
buildpacks/spec#178

[natalieparellano]

Blocked on #530 and (I think?) #471

This issue could probably benefit from being broken down into smaller issues, and having an associated spec PR at least in draft status (maybe the spec PR could also be broken down).

Some thoughts on how this could be chunked based on the existing draft PR to lifecycle (#410):

• Static satisfaction (via stack.toml) of statically required (via buildpack.toml) mixins <- pack does this today, need to add to lifecycle
  * Dynamic satisfaction (via build plan) of dynamically required (via build plan) mixins <- existing draft PR for stack buildpacks does this
  * Dynamic satisfaction (via build plan) of statically required (via buildpack.toml) mixins <- seems this should be easier to add if we've done the first two
• Static satisfaction (via stack buildpacks' buildpack.toml) of statically required (via buildpack.toml) mixins
• Static satisfaction (via stack OR stack buildpacks' buildpack.toml) of dynamically required (via build plan) mixins
• A way for stack buildpacks to disable cache on rebuilds
• A way for stack buildpacks to skip taking new snapshots on subsequent re-runs
• A way for stack buildpacks to exclude/include parts of the snapshot
• extend phase
• Probably other things missing here

@jabrown85 what do you think?

[natalieparellano]

Also might be good to think about which of these issues could be parallelizable.

[natalieparellano]

Closing in favor of https://github.com/buildpacks/lifecycle/issues?q=is%3Aissue+label%3Aepic%2Fstackpacks+