Change defaults that depend on the working dir

[ekcasey]

buildpacks/spec#128

In platform API 0.5, all default file paths that are currently relative to the working directory should changed to be relative to the layers directory.

[yaelharel]

The default paths that should be changed are:

- group.toml
- plan.toml
- analyzed.toml
- project-metadata.toml
- report.toml

[yaelharel]

@ekcasey, does the default directory supposed to be layers or should it be based on the configured layers directory?
For example - if the layers directory was configured to /abc, should the group.toml still be set to /layers/group.toml or should it be /abc/group.toml?

[ekcasey]

@yaelharel It should be based on the configured layers directory.