Reintrodue a piece of sys_path code with test

This piece was thought to not be needed. It turns out it is

jedi/evaluate/sys_path.py

@@ -232,7 +232,11 @@ def transform_path_to_dotted(sys_path, module_path):
         for p in sys_path:
             if module_path.startswith(p):
                 # Strip the trailing slash/backslash
-                rest = module_path[len(p) + 1:]
+                rest = module_path[len(p):]
+                # On Windows a path can also use a slash.
+                if rest.startswith(os.path.sep) or rest.startswith('/'):
+                    # Remove a slash in cases it's still there.
+                    rest = rest[1:]
 
                 if rest:
                     split = rest.split(os.path.sep)

test/test_evaluate/test_sys_path.py

@@ -94,6 +94,8 @@ _s = ['/a', '/b', '/c/d/']
 
         (['/foo', '/foo/bar'], '/foo/bar/baz', ('baz',), False),
         (['/foo/bar', '/foo'], '/foo/bar/baz', ('baz',), False),
+
+        (['/'], '/bar/baz.py', ('bar', 'baz',), False),
     ])
 def test_transform_path_to_dotted(sys_path_, module_path, expected, is_package):
     # transform_path_to_dotted expects normalized absolute paths.