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 login/redirect URIs.
  - Build the request.
  - Set the default base URI.
  - Run normal code.

Since we try to generate URIs before we provide a default, this fatals. Instead, don't try to build objects.

An alternative fix might be to try to set defaults earlier, but we depend on some config and on building the Request in order to be able to figure out if a request is HTTP or HTTPS right now. We could assume one, or guess, or use protocol-relative URIs (`///host.com`), but I think this fix is a little cleaner overall. If we keep hitting similar stuff, we could look into alternate fixes.

We could also set some kind of "setup mode" flag and make `getURI()` if it's called during setup mode to detect these during testing. I'd like to hit one more of these before doing that, though.

Test Plan: Reproduced the issue, applied the patch, verified this fixes it.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8395