Implement Buildpack RFC 0074: Move layer types to new table in layer.toml

[ryanmoran]

Summary

Buildpack RFC0074 makes a modification to the layer metadata file to move the build, launch, and cache keys under a top-level types table. The new format would look like the following:

[types]
  launch = true
  build = false
  cache = false

We will need to make sure that buildpacks using Buildpack API 0.6 or higher have their layer metadata files written to include this new types table.

Proposal

If the buildpack is using Buildpack API 0.6 or higher, include the build, launch, and cache keys under a top-level types table when writing out the layer metadata file.

For buildpacks using older versions of the Buildpack API, keep the existing behavior.