Deprecate calling fixture functions directly

This will now issue a RemovedInPytest4Warning when the user calls
a fixture function directly, instead of requesting it from test
functions as is expected

Fix #3661

src/_pytest/compat.py

@@ -84,6 +84,7 @@ def iscoroutinefunction(func):
 
 
 def getlocation(function, curdir):
+    function = get_real_func(function)
     fn = py.path.local(inspect.getfile(function))
     lineno = function.__code__.co_firstlineno
     if fn.relto(curdir):