Use a default package config for a buildpackage

[jkutner]

Summary

This change causes pack to create a default configuration when no package.toml file is provided to the package-buildpack command. The default is the equivalent of:

[buildpack]
uri = "."

This is important for buildpack authors who want to publish to the Buildpack Registry. In order to publish, you must create a buildpackage, but learning about the package.toml file in order to share your buildpack is an unnecessary hurdle for a simple buildpack.

Output

Before

$ pack package-buildpack my-cnb
ERROR: Please provide a package config path, using --config

After

If the buildpack is a normal buildpack, it just works:

$ pack package-buildpack my-cnb
Successfully created package my-cnb

If the buildpack is a meta-buildpack it will create an error:

$ pack package-buildpack my-meta-cnb
ERROR: saving image: buildpack jkutner/minecraft@0.1.0 references buildpack heroku/jvm@0.1 which is not present

Documentation

• Should this change be documented?
  • Yes, see #TBD
  • No

[dfreilich]

I really like this. I think it helps the UX a lot, and it should make it more similar to pack build, which builds the application dir you are in at the moment.

[dfreilich]

UA for it fails, due to the changes from #926.
Attempting to use it results in:

$  ~/workspace/pack/out/pack package-buildpack hello-moon
ERROR: invalid platform.os specified: DOCKER_OS is linux

[jkutner]

@dfreilich fixed and rebased. thanks