Refactor to support external Checkout subclasses.
Refactor to support external Checkout subclasses.

This allows any Checkout subclasses to be discovered by the Checkout
factory. Previously, calling globals() limited it to classes defined in
this module, which meant it didn't work for classes which couldn't be
added to annotated_checkout directly (e.g. chrome-internal code).

Review URL: https://codereview.chromium.org/12850003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@188497 0039d316-1c4b-4281-b951-d872f2087c98

shishkander/recipes-py,shishkander/recipes-py,luci/recipes-py,luci/recipes-py