Attempt to get name for editable file:// URLs

When installing an editable from file:///path/to/file, pip currently does
not attempt to determine the name from #egg=NAME, just passing back
None.  This causes constraints code to completely ignore this line
resulting in unexpected installation behaviour.

This patch makes '-e file:///path#egg=name' function similarly to
'file:///path#egg=name' and '-e git+URL#egg=name'.  If #egg=name is not
defined, it returns None and the package becomes an unamed requirement,
which constraints will not parse but in the case of a requirement will
later be processed and determined.

Closes #3026

pip/req/req_install.py

@@ -1065,6 +1065,8 @@ def parse_editable(editable_req, default_vcs=None):
         .[some_extra]
     """
 
+    from pip.index import Link
+
     url = editable_req
     extras = None
 
@@ -1086,9 +1088,10 @@ def parse_editable(editable_req, default_vcs=None):
         url_no_extras = path_to_url(url_no_extras)
 
     if url_no_extras.lower().startswith('file:'):
+        package_name = Link(url_no_extras).egg_fragment
         if extras:
             return (
-                None,
+                package_name,
                 url_no_extras,
                 pkg_resources.Requirement.parse(
                     '__placeholder__' + extras
@@ -1096,7 +1099,7 @@ def parse_editable(editable_req, default_vcs=None):
                 {},
             )
         else:
-            return None, url_no_extras, None, {}
+            return package_name, url_no_extras, None, {}
 
     for version_control in vcs:
         if url.lower().startswith('%s:' % version_control):

tests/functional/test_install_reqs.py

@@ -282,7 +282,6 @@ def test_constraints_local_install_causes_error(script, data):
     assert 'Could not satisfy constraints for' in result.stderr
 
 
-@pytest.mark.xfail
 def test_constraints_constrain_to_local_editable(script, data):
     to_install = data.src.join("singlemodule")
     script.scratch_path.join("constraints.txt").write(