[Stacked PR] Schneems/layer output mut

[schneems]

• Pulls in the change from my &mut layer PR (first 2 commits)
• Replaces inline_output with a LayerOutput trait (next commit)
• Adds logging to an example in the examples repos (next commit)

Need

I can get info into the layer now that it is mutable, but the result of context.handle_layer is a LayerData which does not retain a reference to the original layer. One option could be making LayerData take two generics instead of one, but this has large API consequences.

[schneems]

Closing this without merging. We've decided to move forward in a different way:

The section_logger renamed inline_output and in this PR changed to a LayerOutput trait is fundamentally a hack. We could continue with this hack by modifying LayerData in such a way that allows us to retrieve a layer once it's done executing.

When we tried this work, it impacts the internals, it impacts the consumers, and ultimately it seems like a lot of work for something we might not ultimately want.

We talked about:

• Get rid of the hacks i.e. remove inline_output, and don't replace it with anything
• Merge the scoped work in schneems/output
• Continue that work by re-adding in older features that were removed to help make the PR easier to review
• Once that work is done and merged:
  • Implement buildpack logging with this new code in one or more Java buildpacks. These buildpacks do not log inside the layer at all (which, I feel they should, but that's not the main concern.)
  • Work towards a layer interface that allows pushing/pulling data and bubbling up output messages so that we do not need any hacks. This builds off of the work in https://github.com/heroku/buildpacks-ruby/pull/203 (though not 1:1). This work has been known as a "nice to have" but instead of effectively wasting time working on hacks and making the internals of LayerData worse, we're reprioritizing that work. It was already important, now it's more urgent as well.

At the end we'll have a new output and layer concept that go hand in hand. We can update the Ruby buildpack using this style to ensure it's suitable for all buildpacks.