Implementation of Platform API verification

[jromero]

Implements verification according to buildpacks/spec#67.

Caveats:

• I did not create a compatible test for every binary due to the level of scaffolding necessary.
• There isn't any mention of rebaser in the RFC or spec so I left it out. Should it be included?

Notes:

• @ekcasey Can I get extra 👀on the assertions regarding compatiblity here?
  At this time it's not all too relevant given we are pre-stable but would like to document it accurately.
• Stole some cool stuff from Move lifecycle build phases into single binary #232. Nonetheless, we're going to have some merge conflicts.

Resolves #224

[ekcasey]

I feel a little iffy about introducing happy path acceptance tests that don't actually do the full setup etc. I think between the unit tests of the compatibility verification and the unhappy path tests we should be covered.

If you feel strongly that we should have them, we need to do more to control the test inputs (images that will never exist, temp dirs or discard for all relevant directories).

[ekcasey]

@jromero I think rebaser should be included. It should be part of the platform API despite the fact that the platform API is incompletely specified.

[zmackie]

Accepted with:

± zm |master U:1 ✗| → docker run -it -e CNB_PLATFORM_API=0.1 cnb/sample:accept-lifecycle-0.0.6
cnb@7a46687c39a6:/layers$ /lifecycle/detector
ERROR: failed to : the Lifecycle's Platform API version is 0.2 which is incompatible with Platform API version 0.1
cnb@7a46687c39a6:/layers$ exit
  |2.6.3| NY-Floater-15565 in ~/workspace/samples
± zm |master U:1 ✗| → docker run -it -e CNB_PLATFORM_API=0.2 cnb/sample:accept-lifecycle-0.0.6
cnb@5d6c4e2002ea:/layers$ /lifecycle/detector
ERROR: No buildpack groups passed detection.
ERROR: Please check that you are running against the correct path.
ERROR: failed to detect: no buildpacks participating
cnb@5d6c4e2002ea:/layers$ exit
  |2.6.3| NY-Floater-15565 in ~/workspace/samples
± zm |master U:1 ✗| → docker run -it -e CNB_PLATFORM_API=0.3 cnb/sample:accept-lifecycle-0.0.6
cnb@e252b0b054a7:/layers$ /lifecycle/detector
ERROR: failed to : the Lifecycle's Platform API version is 0.2 which is incompatible with Platform API version 0.3
cnb@e252b0b054a7:/layers$ exit
  |2.6.3| NY-Floater-15565 in ~/workspace/samples
± zm |master U:1 ✗| → docker run -it -e CNB_PLATFORM_API= cnb/sample:accept-lifecycle-0.0.6
cnb@90d6bce17f99:/layers$ /lifecycle/detector
ERROR: No buildpack groups passed detection.
ERROR: Please check that you are running against the correct path.
ERROR: failed to detect: no buildpacks participating
cnb@90d6bce17f99:/layers$ exit
  |2.6.3| NY-Floater-15565 in ~/workspace/samples
± zm |master U:1 ✗| → docker run -it -e CNB_PLATFORM_API='' cnb/sample:accept-lifecycle-0.0.6
cnb@97c404171261:/layers$ /lifecycle/detector
ERROR: No buildpack groups passed detection.
ERROR: Please check that you are running against the correct path.
ERROR: failed to detect: no buildpacks participating
cnb@97c404171261:/layers$ exit