Make resolvelib's provider capable of handling empty iterators

This is _technically_ possible with the API, and accounting for that
enables the resolver to evolve with this information.

Author: pradyunsg

src/pip/_internal/resolution/resolvelib/provider.py

@@ -124,14 +124,29 @@ def get_preference(  # type: ignore
         * If equal, prefer if any requirement is "pinned", i.e. contains
           operator ``===`` or ``==``.
         * If equal, calculate an approximate "depth" and resolve requirements
-          closer to the user-specified requirements first.
+          closer to the user-specified requirements first. If the depth cannot
+          by determined (eg: due to no matching parents), it is considered
+          infinite.
         * Order user-specified requirements by the order they are specified.
         * If equal, prefers "non-free" requirements, i.e. contains at least one
           operator, such as ``>=`` or ``<``.
         * If equal, order alphabetically for consistency (helps debuggability).
         """
-        lookups = (r.get_candidate_lookup() for r, _ in information[identifier])
-        candidate, ireqs = zip(*lookups)
+        try:
+            next(iter(information[identifier]))
+        except StopIteration:
+            # There is no information for this identifier, so there's no known
+            # candidates.
+            has_information = False
+        else:
+            has_information = True
+
+        if has_information:
+            lookups = (r.get_candidate_lookup() for r, _ in information[identifier])
+            candidate, ireqs = zip(*lookups)
+        else:
+            candidate, ireqs = None, ()
+
         operators = [
             specifier.operator
             for specifier_set in (ireq.specifier for ireq in ireqs if ireq)
@@ -146,11 +161,14 @@ def get_preference(  # type: ignore
             requested_order: Union[int, float] = self._user_requested[identifier]
         except KeyError:
             requested_order = math.inf
-            parent_depths = (
-                self._known_depths[parent.name] if parent is not None else 0.0
-                for _, parent in information[identifier]
-            )
-            inferred_depth = min(d for d in parent_depths) + 1.0
+            if has_information:
+                parent_depths = (
+                    self._known_depths[parent.name] if parent is not None else 0.0
+                    for _, parent in information[identifier]
+                )
+                inferred_depth = min(d for d in parent_depths) + 1.0
+            else:
+                inferred_depth = math.inf
         else:
             inferred_depth = 1.0
         self._known_depths[identifier] = inferred_depth