R1:cdeea11fd3f4

Don't actually construct auth providers when checking for their existence

Summary:
A user reported this stack trace:

http://pastebin.com/6auGbZsE

...on this GitHub issue:

https://github.com/facebook/phabricator/issues/389#issuecomment-36612511

The problem is similar to the original report, but not identical. In this case, we're following a sequence of steps like:

- Run setup checks.
- Check for enabled providers, in order to raise "no providers configured yet" warning.
- Try to generate…