The `stack` property lived on the `buildpacks` plugin but was shared by both the herokuish and pack builders, so a value valid for one builder broke the other. Each builder now owns its own `stack` property managed via `builder-herokuish:set` and `builder-pack:set`, while `buildpacks:set-property stack` is deprecated and forwards the value to the builder detected from it; existing values migrate automatically on upgrade and changing a builder's stack clears that builder's cache.
This change refactors the compiled golang plugins into 3 distinct binaries:
- commands: already existing
- subcommands/subcommands: entrypoint into all subcommands
- triggers: entrypoint into all triggers
It then further symlinks triggers and subcommands to the built binaries. This results in both a much faster build process as well as smaller package size.
