[lib] Extract getRelativeMemberInfos
[lib] Extract getRelativeMemberInfos

Summary: In the following diff, I'm modifying `ThreadInfo.members` from `MemberInfo` to `RelativeMemberInfo` so the username is available there. I want to extract this function so I can use it in the following diff.

Test Plan: Flow (no changes are made here, the function is just extracted)

Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1785

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal