Migrate PullRequestResource to go-scm.

This changes the underlying implementation of the PullRequestResource
to jenkins-x/go-scm in order to take advantage of the repo drivers to
support other SCM providers.

In the process, this also allowed for refactoring to have a cleaner
distinction between interaction with SCM APIs and converting the data to
an on disk format.

BREAKING CHANGES:
* go-scm does not yet support updating comments, so that behavior is
removed for the time being since:
  1) it's unclear if anyone is actively
  using it.
  2) this behavior can be roughly replicated with a delete +
  create.
  3) the benefits of having the other SCM providers outweighs this
  feature.

Fixes #1066.

cmd/pullrequest-init/README.md

@@ -13,18 +13,3 @@ Currently supported providers:
 ## Generic pull request payload
 
 For information about the payloads written to disk, see the [resource documentation](../../docs/resources.md#pull-request-resource).
-
-### Status code conversion
-
-Tekton Status Code | GitHub Status State
------------------- | -------------------
-success            | success
-neutral            | success
-queued             | pending
-in_progress        | pending
-failure            | failure
-unknown            | error
-error              | error
-timeout            | error
-canceled           | error
-action_required    | error