Enable package and offline_package to support nested project structures

[dmikusa]

What happened?

• What were you attempting to do?

The current implementation assumes that one source repo will have one buildpack. If you have multiple buildpacks in a source repo then this won't work.

This is an enhancement request to add a source path property that would point to a sub path of the repository. The jobs should then run from the root of that sub path, instead of the root of the source repo. The default would be to continue to run from the source repo root.

Checklist

• I have included log output.
• The log output includes an error message.
• I have included steps for reproduction.

[dmikusa]

Closed by #548