[lib] Update all-but-one instance of RelativeMemberInfo with RelativeMemberInfo|MinimallyEncodedRelativeMemberInfo

Summary:
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