This allows build to succeed at installing the extensions, so that it's
possible to debug changes.

The wrinkles here are that:
1. SDK's before the RC one don't know how to install on RC.
2. The RC SDK *requires* a <Prerequisites> element
3. The Willow installer fails to install vsixes that have a <Prerequisites> element.

To workaround 2 and 3, if we detect that you are building with something before RC,
we run an xsl transform to remove the <Prerequisites> element before the SDK uses it.

Since our official builds for insertion into Willow are still on VS 2015, they won't
have the element.