Avoid modifying path placeholders created by editable installs

The setuptools implementation of editable installs will insert a
placeholder entry into sys.path as part of its magic to register its
custom import mechanism.

These are not real filesystem paths and as such should not be
rewritten to absolute paths.

changelog/1028.bugfix

@@ -0,0 +1 @@
+Fix compatiblity issue between `looponfail` and editable installs.

src/xdist/looponfail.py

@@ -160,7 +160,8 @@ def init_worker_session(channel, args, option_dict):
     newpaths = []
     for p in sys.path:
         if p:
-            if not os.path.isabs(p):
+            # Ignore path placeholders created for editable installs
+            if not os.path.isabs(p) and not p.endswith(".__path_hook__"):
                 p = os.path.abspath(p)
             newpaths.append(p)
     sys.path[:] = newpaths