change imports to avoid shadowing

in
```
from buildbot.util import poll

class Foo:
    @poll.method
    def someMethod(self):
        pass
    def poll(self):
        pass
```

The `poll` in `@poll.method` correctly refers to the module.  However,
if the order of the methods is reversed, it refers instead to the `poll`
method in the class scope.

Pyflakes only detects this in the problematic ordering, while pylint
detects it even in the working order.  Pylint is probably right -
re-ordering methods shouldn't change behavior!

master/buildbot/changes/base.py

@@ -18,8 +18,8 @@
 from zope.interface import implements
 
 from buildbot.interfaces import IChangeSource
-from buildbot.util import poll
 from buildbot.util import service
+from buildbot.util.poll import method as poll_method
 
 
 class ChangeSource(service.ClusteredService):
@@ -57,15 +57,15 @@ def __init__(self, name=None, pollInterval=60 * 10, pollAtLaunch=False):
         self.pollInterval = pollInterval
         self.pollAtLaunch = pollAtLaunch
 
-    @poll.method
+    def poll(self):
+        pass
+
+    @poll_method
     def doPoll(self):
         d = defer.maybeDeferred(self.poll)
         d.addErrback(log.err, 'while polling for changes')
         return d
 
-    def poll(self):
-        pass
-
     def force(self):
         self.doPoll()
 

master/buildbot/test/unit/test_util_poll.py

@@ -13,15 +13,15 @@
 #
 # Copyright Buildbot Team Members
 
-from buildbot.util import poll
+from buildbot.util.poll import method as poll_method
 from twisted.internet import defer
 from twisted.internet import task
 from twisted.trial import unittest
 
 
 class TestPollerSync(unittest.TestCase):
 
-    @poll.method
+    @poll_method
     def poll(self):
         self.calls += 1
         if self.fail:
@@ -107,7 +107,7 @@ def test_fails(self):
 
 class TestPollerAsync(unittest.TestCase):
 
-    @poll.method
+    @poll_method
     def poll(self):
         assert not self.running, "overlapping call"
         self.running = True