Add support for Single repository & branch mapping in Multibranch pipelines

[Zetym]

When mapping up a Plastic repository/branch as a Single repository & branch in a Multibranch pipeline, the controller will do a full checkout of the repository for the polling even though a single pipeline does a lightweight checkout. This PR enables the controller to perform lightweight checkouts even when mapped as a single repository & branch in a Multibranch pipeline. This doesn't try to provide full Multibranch support as requested in #32 , but it enables various workarounds to use them.

When determining if the controller should do a lightweight checkout, the Jenkins asks the SCM if it can do lightweight checkouts. In a single pipeline job that question goes directly to this plugin through the supports(SCM scm) method, but in a multibranch Plastic gets wrapped in a SingleSCMSource that gets the question and forwards it to Plastic, but to the supports(SCMSource source) method. This change replicates the behaviour of the first method if the source provided in the second is a SingleSCMSource wrapper.

Testing done

This change has been running internally for two multibranch projects with several branches mapped each for a couple months now without issue. Before implementing this change the controller ran out of disk space immediately on trying to poll all the branches.

Submitter checklist

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[SRombautsU]

Thanks a lot for taking the time to contribute this change back to us :) I reviewed the code quickly and thanks to your explanation it seems good to me I'll discuss this with the maintainer next week so we can merge this asap!