Move conflict warning to just-before success message

This is a much better location for these errors, since they're in a much
more visible spot. We've had reports in the past of users missing these
messages, and changing where we present these warnings should help
resolve that issue.

We do lose the ability for an advanced user to potentially see the
warning and abort installation before the conflicts are introduced, but
given that we don't even pause for input, I don't think that's a strong
argument and neither do I view this as necessary.

src/pip/_internal/commands/install.py

@@ -373,13 +373,13 @@ def run(self, options, args):
             )
 
             # Check for conflicts in the package set we're installing.
+            conflicts = None  # type: Optional[ConflictDetails]
             should_warn_about_conflicts = (
                 not options.ignore_dependencies and
                 options.warn_about_conflicts
             )
             if should_warn_about_conflicts:
                 conflicts = self._determine_conflicts(to_install)
-                self._warn_about_conflicts(conflicts)
 
             # Don't warn about script install locations if
             # --target has been specified
@@ -421,6 +421,10 @@ def run(self, options, args):
                 except Exception:
                     pass
                 items.append(item)
+
+            if conflicts is not None:
+                self._warn_about_conflicts(conflicts)
+
             installed_desc = ' '.join(items)
             if installed_desc:
                 write_output(