In a document without references, the default group doesn't exist.
Make sure that we don't assume any group's existence.
Bug: T373661
Change-Id: I1bfd41b0b61938f32219d61142cd576c8ca97ace
Looking for other references can (as of now) never work in source
mode. There are never other references or reference lists.
Bug: T373661
Change-Id: Icaad4e71a1538c5fad6a0f016b342a7cb7b9db2f
Pushes per-group knowledge down into a structured object and give it
an interface, separated from the singleton cache across all groups.
Also changes the behavior of orphaned subrefs so that they're still
rendered as subrefs, with an error placeholder where the parent
should be.
Bug: T372871
Change-Id: I84e679a8365f3fbfabaf344d99f56f6d069c0776