Add support for docker URI images in buildpackstore.

[robdimsdale]

Summary / Use Cases

This PR adds support for docker/OCI images in the buildpackstore. This allows us to use pre-built images as dependent buildpacks instead of having to build them from scratch each time.

Using OCI images as dependencies has multiple advantages:

1. It is more correct, as we're testing against the actual buildpacks that have been created, rather than building from scratch.
2. Extending the above, point, it is trivially-easy to pin the dependent buildpacks to known versions. Currently that is quite difficult (potentially impossible) in occam/freezer.
3. It is more efficient and stable (as we can leverage the docker cache and not worry about accidentally corrupting the freezer cache)

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[robdimsdale]

This is kind of gross, but it's the only way I could see to make this backwards-compatible and avoid a breaking change in the exported function signatures.

The OCI fetcher has to be injected, so either we force the user to inject it at construction time (i.e. NewBuildpackStore(dockerPull BPStoreDockerPull)) which is a breaking change, or we accept that it might be nil when we try to use it.

[robdimsdale]

After chatting with @ForestEckhardt I think we can close this, and instead require buildpack authors to explicitly switch to occam.Docker.Pull in the integration tests when they switch to a docker image in the integration config.

This abstraction is probably unnecessary.