Fix concretize bug where provider sort couldn't handle version ranges

lib/spack/spack/concretize.py

@@ -95,7 +95,11 @@ def cmp_externals(a, b):
                                      not b.external and b.external_module):
                 # We're choosing between different providers, so
                 # maintain order from provider sort
-                return candidates.index(a) - candidates.index(b)
+                index_of_a = next(i for i in range(0, len(candidates)) \
+                                  if a.satisfies(candidates[i]))
+                index_of_b = next(i for i in range(0, len(candidates)) \
+                                  if b.satisfies(candidates[i]))
+                return index_of_a - index_of_b
 
             result = cmp_specs(a, b)
             if result != 0: