Expanded to cover MatrixBuild, not just MatrixRun.

For example, a common use cases are:

- skip the parent checkout and only do the checkout in MatrixRun
- check out only once in MatrixBuild and then let all MatrixRuns build from the same directory

Accoringly,

- renamed classes to drop 'Run' since now they cover 'Build' as well.
- instead of using the boolean return value to indicate the fallback to
  default, using some backdoor to have the default implementation do
  this. I think this keeps the abstraction cleaner for the implementor
  of the extension point.