Enable import-error check on //testing

Enables the import-error pylint check on //testing. Most errors are
fixed by updating the PYTHONPATH used by pylint to include the necessary
directories, although some import errors are simply suppressed (most
commonly for vpython-provided modules).

Also takes this opportunity to add comments about where each block of
non-standard imports comes from. This was already present in a number of
places, but not everywhere.

Bug: 353942917
Change-Id: I506d35e5a6ae56471618e6b4af57dd2ecb64dd7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6092016
Commit-Queue: Brian Sheedy <[email protected]>
Reviewed-by: Ben Pastene <[email protected]>
Reviewed-by: Paul Semel <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1396194}
diff --git a/testing/buildbot/buildbot_json_magic_substitutions_unittest.py b/testing/buildbot/buildbot_json_magic_substitutions_unittest.py
index 3426494..e07e6d9 100755
--- a/testing/buildbot/buildbot_json_magic_substitutions_unittest.py
+++ b/testing/buildbot/buildbot_json_magic_substitutions_unittest.py
@@ -5,6 +5,7 @@
 
 import unittest
 
+# //testing/buildbot imports.
 import buildbot_json_magic_substitutions as magic_substitutions