Fix a few issues with the "import symbols" script

Summary: Ref T7977. Fix a few issues that I forgot to fix up.

Test Plan: Run the script.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7977

Differential Revision: https://secure.phabricator.com/D12668