import-checker: allow symbol imports from typing module As we add typing annotations, we'll want to use a lot of symbols from the `typing` module. Typing `typing` all the time will be annoying. Let's allow symbol imports from this module. While I was here, I changed some comments from "whitelist" to "allow list" as the former is non-inclusive terminology. Differential Revision: https://phab.mercurial-scm.org/D12365

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