python: Don't assume SimObjects live in the global namespace

The importer in Python 3 doesn't like the way we import SimObjects
from the global namespace. Convert the existing SimObject declarations
to import from m5.objects. As a side-effect, this makes these files
consistent with configuration files.

Change-Id: I11153502b430822130722839e1fa767b82a027aa
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15981
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/mem/cache/prefetch/Prefetcher.py

@@ -39,12 +39,13 @@
 # Authors: Ron Dreslinski
 #          Mitch Hayenga
 
-from ClockedObject import ClockedObject
-from IndexingPolicies import *
 from m5.SimObject import *
 from m5.params import *
 from m5.proxy import *
-from ReplacementPolicies import *
+
+from m5.objects.ClockedObject import ClockedObject
+from m5.objects.IndexingPolicies import *
+from m5.objects.ReplacementPolicies import *
 
 class HWPProbeEvent(object):
     def __init__(self, prefetcher, obj, *listOfNames):