[lib] Convert getPotentialMemberItems into a hook

Summary:
This should be a no-op change in terms of the results that getPotentialMemberItems used to return. I'm simply converting it into a hook so I can call a hook that will construct a search index in the next diff, necessary to make it possible to prefix-search for users by their ENS names.

Addresses ENG-5402

Test Plan: Did some manual testing to confirm that there was no change in the search results returned (both by console.log'ing before and after, as well as searching for users on the app)

Reviewers: atul, ginsu, inka

Reviewed By: ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10229