Combine exporter and cacher phases

[lukasberger]

Combines the exporter and cacher phases to reuse layer tarballs.

• Move logic from cacher.go to exporter.go.
• Move tests from cacher_test.go to exporter_test.go.
• Rename cache image flag from '-image' to '-cache-image'.
• Rename cache directory flag from '-path' to '-cache-dir'.
• Remove unused 'In' field of the Exporter struct.
• Remove cacher implementation, tests, and command.
• Upgrade Platform API version to 0.2 as this PR contains breaking changes.

Part of the implementation of buildpacks/rfcs#21.

Signed-off-by: Lukas Berger [email protected]

[jkutner]

This will mean a breaking change for pack, which is probably fine, but we could consider avoiding it or allowing a grace period (maybe until we can combine it with other breaking changes). We could do the following methinks:

• Leave the cacher in place
• Allow -path on restorer but ignore it (or use it when -cache-dir isn't provided)
• Because pack won't pass -cache-image to exporter it won't create the cache

I think we're probably still at an ok place to make breaking changes, but if this were to be the only breaking in change in a release I would want us to consider ^

[ekcasey]

@lukasberger I approved this and #207. This one could use a tiny bit of cleanup once it can be rebased on #207 (see comments).

I think we need pack to work with platform API 0.2 before we can merge either of these to master or we will lose the ability to test master of lifecycle with pack. You mentioned you were working on those pack changes in parallel with these lifecycle changes. Do you plan to submit that PR or should we find somebody else to do it?

[lukasberger]

@ekcasey The pack changes were merged in buildpacks/pack#384. Depending on how everyone else feels about the flag name cleanup that @jromero proposed being part of this PR, I can make the changes in pack as well. Regarding the cleanup, did you mean after #207 is merged, or do you want me to rebase this PR on top of the other branch?

[ekcasey]

@lukasberger Now that I realize the changes are already merge in pack I am inclined to merge both of these at once (merging just one of them will put us in a breaking state).

After they are merged you (or another contributor) can open a PR refactoring cmd/exporter/main.go to use cache.NewImageCacheFromName from the other PR.