Repurpose backtracking hook

See discussion at pypa/pip#10937.

src/resolvelib/reporters.py

@@ -36,7 +36,7 @@ def resolving_conflicts(self, causes):
         :param causes: The information on the collision that caused the backtracking.
         """
 
-    def backtracking(self, candidate):
+    def backtracking(self, criterion, candidate):
         """Called when rejecting a candidate during backtracking."""
 
     def pinning(self, candidate):

src/resolvelib/reporters.pyi

@@ -6,6 +6,6 @@ class BaseReporter:
     def ending_round(self, index: int, state: Any) -> Any: ...
     def ending(self, state: Any) -> Any: ...
     def adding_requirement(self, requirement: Any, parent: Any) -> Any: ...
-    def backtracking(self, candidate: Any) -> Any: ...
+    def backtracking(self, criterion: Any, candidate: Any) -> Any: ...
     def resolving_conflicts(self, causes: Any) -> Any: ...
     def pinning(self, candidate: Any) -> Any: ...

src/resolvelib/resolvers.py

@@ -212,6 +212,7 @@ def _attempt_to_pin_criterion(self, name):
             try:
                 criteria = self._get_updated_criteria(candidate)
             except RequirementsConflicted as e:
+                self._r.backtracking(e.criterion, candidate)
                 causes.append(e.criterion)
                 continue
 
@@ -281,8 +282,6 @@ def _backtrack(self):
             # Also mark the newly known incompatibility.
             incompatibilities_from_broken.append((name, [candidate]))
 
-            self._r.backtracking(candidate=candidate)
-
             # Create a new state from the last known-to-work one, and apply
             # the previously gathered incompatibility information.
             def _patch_criteria():