Add info about buildpack used to staging response.

The goal is to get information about the buildpack used for staging
(including detected admin buildpacks) back to the CC so it can be used
to populate app usage events.

Highlights:
* Add path of buildpack used for staging to staging_info.yml
* Refactor buildpack manager out of the staging workspace
  - Staging task creates and holds a reference to the buildpack manager
  - Buildpack manager passed to workspace prepare
  - Consolidated builpack ordering tests in buildpack manager
* Add methods to buildpack manager to return buildpack key or a system
  buildpack url from a path
* Return the buildpack url (system or custom) and the buildpack key in
  the staging complete message