Present found conflicts when discarding some criterion

Fixes gh-9254.
Closes gh-10258.
See #10258 (comment) for inspiration.

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

@@ -32,6 +32,19 @@ def __init__(self) -> None:
             ),
         }
 
+    def discarding_conflicted_criterion(self, criterion, candidate) -> None:
+        msg = "Will try a different candidate, due to conflict:"
+        for req_info in criterion.information:
+            req, parent = req_info.requirement, req_info.parent
+            # Inspired by Factory.get_installation_error
+            msg += "\n    "
+            if parent:
+                msg += f"{parent.name} {parent.version} depends on "
+            else:
+                msg += "The user requested "
+            msg += req.format_for_error()
+        logger.info(msg)
+
     def backtracking(self, candidate: Candidate) -> None:
         self.backtracks_by_package[candidate.name] += 1
 

src/pip/_vendor/resolvelib/reporters.py

@@ -30,6 +30,9 @@ def adding_requirement(self, requirement, parent):
             requirements passed in from ``Resolver.resolve()``.
         """
 
+    def discarding_conflicted_criterion(self, criterion, candidate):
+        """Called when discarding conflicted requirements."""
+
     def resolving_conflicts(self, causes):
         """Called when starting to attempt requirement conflict resolution.
 

src/pip/_vendor/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.discarding_conflicted_criterion(e.criterion, candidate)
                 causes.append(e.criterion)
                 continue