style: use import git_trace2_event_log_base everywhere

Align with Google style guides and `import git_trace2_event_log_base` as
a module rather than import symbols out of it.

Change-Id: I98c68a9ddca06325a8e0a620346c408e1ff3b661
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/654816
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Brian Gan <brgan@google.com>

git_command.py

@@ -21,7 +21,7 @@ import sys
 from typing import Any, Optional
 
 import error
-from git_trace2_event_log_base import BaseEventLog
+import git_trace2_event_log_base
 import platform_utils
 import repo_logging
 import repo_trace
@@ -347,7 +347,7 @@ class GitCommand:
         command.extend(cmdv[1:])
 
         event_log = (
-            BaseEventLog(env=env, add_init_count=True)
+            git_trace2_event_log_base.BaseEventLog(env=env, add_init_count=True)
             if add_event_log
             else None
         )