flake8: fix nested import checks

Import modules at the top of the module in most cases.  Many of these
weren't actually saving any time:
* editor: shlex is already imported by the main module
* platform_utils: On Windows, the win32 module is almost always needed
* tests: There's no point in optimizing these.

The trace import in the main module is about the only one worth allowing
since that module itself is rarely used, and only when in a specific
debug mode.

Change-Id: I5d3fcecbe51e0ce78d4e856d7033565150ef4526
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/654103
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

editor.py

@@ -14,6 +14,7 @@
 
 import os
 import re
+import shlex
 import subprocess
 import sys
 import tempfile
@@ -89,8 +90,6 @@ least one of these before using this command.""",  # noqa: E501
 
             if platform_utils.isWindows():
                 # Split on spaces, respecting quoted strings
-                import shlex
-
                 args = shlex.split(editor)
                 shell = False
             elif re.compile("^.*[$ \t'].*$").match(editor):