Split schedulers into their own submodule.

xdist/report.py

@@ -0,0 +1,26 @@
+import py
+from difflib import unified_diff
+
+
+def report_collection_diff(from_collection, to_collection, from_id, to_id):
+    """Report the collected test difference between two nodes.
+
+    :returns: detailed message describing the difference between the given
+    collections, or None if they are equal.
+    """
+    if from_collection == to_collection:
+        return None
+
+    diff = unified_diff(
+        from_collection,
+        to_collection,
+        fromfile=from_id,
+        tofile=to_id,
+    )
+    error_message = py.builtin._totext(
+        'Different tests were collected between {from_id} and {to_id}. '
+        'The difference is:\n'
+        '{diff}'
+    ).format(from_id=from_id, to_id=to_id, diff='\n'.join(diff))
+    msg = "\n".join([x.rstrip() for x in error_message.split("\n")])
+    return msg