Bug with constraint solving default

[SeanTAllen]

In constraint.pony:

      try
        Constraints._parse_constraints(version)?
        "main" // Is a constraint: use main until update.
      else

that default, will fail entirely if the branch doesn't exist. Given that GitHub is making it easy to switch default branch names, this is highly problematic. Our tests for constraints fail if the repo being pulled doesn't have said branch. Which is problematic given the constraint exists. This is a bug that should be fixed.

To reproduce, change that "main" to a non-existent branch in our corral-test-repo (e.g. free-candy-branch) and run the integration tests.