tweak stdlib imports to follow Google style guide

Google Python style guide says to import modules.
Clean up all our stdlib imports.  Leave the repo ones alone
for now as that's a much bigger shave.

Change-Id: Ida42fc2ae78b86e6b7a6cbc98f94ca04b295f8cc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/383714
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/abandon.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from collections import defaultdict
+import collections
 import functools
 import itertools
 import sys
@@ -88,8 +88,8 @@ It is equivalent to "git branch -D <branchname>".
 
     def Execute(self, opt, args):
         nb = args[0].split()
-        err = defaultdict(list)
-        success = defaultdict(list)
+        err = collections.defaultdict(list)
+        success = collections.defaultdict(list)
         aggregate_errors = []
         all_projects = self.GetProjects(
             args[1:], all_manifests=not opt.this_manifest_only