[lib] Update all-but-one instance of RelativeMemberInfo with RelativeMemberInfo|MinimallyEncodedRelativeMemberInfo
Replace almost all instances of RelativeMemberInfo with RelativeMemberInfo | MinimallyEncodedRelativeMemberInfo.
We still need to update getRelativeMember, but breaking that into a separate diff since that's more than just updating signature.
Test Plan: Trusting flow.
Reviewers: ashoat, ginsu, tomek, rohan
Reviewed By: ashoat
Differential Revision: https://phab.comm.dev/D9927