Add `-layers` flag to the rebaser

[yaelharel]

We think it's a mistake that the rebaser isn't getting <layers> as an input. We should probably add it to this section.

[yaelharel]

If this issue will be closed after https://github.com/buildpacks/lifecycle/issues/393, we should make the relevant changes to the rebaser.

[ekcasey]

Because rebaser doesn't accept a -layers flag I think we should probably just back to a non-relative default path. The question is, which path?

Before API 0.5, the default was previously /layers/report.toml, which is admittedly a little weird given that there is no other use for the /layers dir during rebase. However this is a path that exists in a builder and in the lifecycle images and it is consistent with export so I think it is probably our best option.

[yaelharel]

@ekcasey, why can't we just add the -layers flags to the rebaser?

[ekcasey]

@ekcasey, why can't we just add the -layers flags to the rebaser?

We could. I was initially hesitant because the rebaser the rebaser has no other use for -layers and given that, the name feels confusing.

I guess with the upcoming addition of stackpacks this is probably the right call (the extender will eventually create layers that must be be passed to the rebaser in the layers dir).

[natalieparellano]

I'm going to close this one as -report seems to solve the platform need here for customizing rebase behavior.