style: use import pager everywhere

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

Change-Id: I20ec5024ece35ae27bf46364e97c59400aae0cf0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/654808
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

main.py

@@ -51,8 +51,7 @@ import git_command
 from git_config import RepoConfig
 from git_trace2_event_log import EventLog
 from manifest_xml import RepoClient
-from pager import RunPager
-from pager import TerminatePager
+import pager
 import repo_trace
 from subcmds import all_commands
 from subcmds.version import Version
@@ -445,7 +444,7 @@ class _Repo:
                 if use_pager is None:
                     use_pager = cmd.WantPager(copts)
             if use_pager:
-                RunPager(config)
+                pager.RunPager(config)
 
         start = time.time()
         cmd_event = cmd.event_log.Add(name, event_log.TASK_COMMAND, start)
@@ -959,7 +958,7 @@ def _Main(argv):
             print("fatal: %s" % e, file=sys.stderr)
             result = 128
 
-    TerminatePager()
+    pager.TerminatePager()
     sys.exit(result)