Rename filescope to loadfile

This makes it consistent with the option (--dist=loadfile) and the other
scheduling file (loadscope.py)

xdist/scheduler/__init__.py

@@ -1,4 +1,4 @@
 from xdist.scheduler.each import EachScheduling  # noqa
 from xdist.scheduler.load import LoadScheduling  # noqa
+from xdist.scheduler.loadfile import LoadFileScheduling  # noqa
 from xdist.scheduler.loadscope import LoadScopeScheduling  # noqa
-from xdist.scheduler.filescope import LoadFileScheduling  # noqa

xdist/scheduler/loadfile.py

@@ -1,4 +1,4 @@
-from . import LoadScopeScheduling
+from .loadscope import LoadScopeScheduling
 from py.log import Producer