Modifications to enable libcnb to be compatible with both versions 0.5 and 0.6 of the API

[dmikusa]

At the moment, the latest version of libcnb (1.20.0) breaks compatibility for buildpacks that are still on the 0.5 version of the API. This patch creates compatibility for both 0.5 and 0.6.

The use case for this change is upgrading buildpacks. On the Paketo team, we have quite a few buildpacks that are using libpak and libcnb. This abrupt change means we either have to hold back libcnb updates or we have to go through and update all of the buildpacks to use the new API. Having compatibility for both version 0.5 and 0.6 eases the transition as we can keep updating libcnb and migrate buildpacks to 0.6 at a more leisurely pace.

My suggestion would be to remove this 0.5 compatibility layer when we add support for the 0.7 (or whatever's next) API version. That way we're supporting the latest two API versions at any given moment, again helping buildpack authors to transition more easily.

[sambhav]

There were some additional attributes added to launch.toml like the default flag for processes. Should we add a warning or throw an error if someone sets that with the v0.5 api?

[dmikusa]

There were some additional attributes added to launch.toml like the default flag for processes. Should we add a warning or throw an error if someone sets that with the v0.5 API?

I added logger.Info("WARNING: Launch layer is setting default=true, but that is not supported until API version 0.6. This setting will be ignored.") to run if any processes are set with Default=true. This check happens when it writes the launch.toml file.