Rename sdkMemberRef to sdkMemberVariantDep

Previously, there were two types sdkMemberRef and sdkMember that seemed
as though the former was referring to the latter but that was not the
case. The former was actually information about the dependency the sdk
had on a specific member variant. The latter is all the information
about a specific member, including all its variants.

This change attempts to clarify that relationship, both through a
better type name and through some improved documentation.

Bug: 186290299
Test: m nothing
Change-Id: I1dcc0ddf3333da9e797bb23ae945d04729839014