Don't instantiate abstract classes in ArcanistConfiguration::buildAllWorkflows

Summary:
I wanted to write a couple of workflows that shared some code in an abstract superclass, but ##arc## would fail trying to instantiate the abstract class.

As it turns out, we can modernize the ##buildAllWorkflows## function a bit, and it will only load concrete objects.

Test Plan:
1. Define an abstract subclass of ##ArcanistBaseWorkflow##.
2. ##arc liberate## the source directory that contains it.
3. Try any ##arc## operation without this diff, and see it fail.
4. Patch this diff and see that ##arc## operations work now.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6324