Merge cache and launch directories

[sclevine]

Specifies #23.

Note: this further simplifies the initial proposal in #23 such that layers have three properties:

1. cache
2. build
3. launch

Such that cached launch layers are fully cache-coherent and always restored.

[nebhale]

A couple of personal clarifications, but the big one is that I think the metadata can be simplified.

[jkutner]

It's not necessarily specific to this change, but I'd prefer the bin/build signature be the following:

/bin/build <layers[EIC]> <platform[AR]> <plan[E]> 

I believe this order puts the most significant arguments first, and improves the ergonomics of the API for the majority of buildpack authors (i.e. most buildpacks won't use plan and many won't need platform, which we know from looking at existing v2 buildpacks that don't use the env dir).

There is nothing mechanically preventing a buildpack author from skipping over platform and plan, but putting them earlier forces an author to think about those arguments. It also makes introductory guides (which will likely not use platform and plan) a little hokey since they have to:

APP_DIR="$(pwd)"
LAYERS_DIR="$3"

It begs the question "what happened to $1 and $2" pretty early on when we're trying to ignore those args.

If this is accepted, bin/develop should match of course.

[nebhale]

@jkutner I strongly disagree and think that value of the consistency in argument position (both detect and build need these arguments) is more valuable.

[jkutner]

I've approved this as-is, but would like to see #29 incorporated.

[nebhale]

Just a heads up as we do the merge into lifecycle, that the spec is still waiting on a couple of updates based on feedback.

[ekcasey]

lifecycle changes implemented in buildpacks/lifecycle#45