Make FolderComputation more extendable

[rsandell]

See downstream jenkinsci/branch-api-plugin#449

Proposed changelog entries

• Entry 1: Issue, Human-readable Text
• ...

Submitter checklist

• JIRA issue is well described
• Changelog entry appropriate for the audience affected by the change (users or developer, depending on the change).
  * Use the Internal: prefix if the change has no user-visible impact (API, test frameworks, etc.)
• Appropriate autotests or explanation to why this change has no tests

[jglick]

Evidently abandoned.