[issue 1548] Make pip install -e uninstall existing versions

by adding an additional call to `requirement.check_if_exists()` to
RequirementSet.install

Fixes pypaGH-1548

pip/req.py

@@ -801,6 +801,10 @@ def check_if_exists(self):
                 return True
             else:
                 self.satisfied_by = pkg_resources.get_distribution(self.req)
+
+            if self.editable and self.satisfied_by:
+                self.conflicts_with = self.satisfied_by
+                return True
         except pkg_resources.DistributionNotFound:
             return False
         except pkg_resources.VersionConflict:
@@ -1368,6 +1372,8 @@ def install(self, install_options, global_options=(), *args, **kwargs):
                         # distribute wasn't installed, so nothing to do
                         pass
 
+                requirement.check_if_exists()
+
                 if requirement.conflicts_with:
                     logger.notify('Found existing installation: %s'
                                   % requirement.conflicts_with)