Fix issue with referencing field in doubly-nested join inside a nest

[christopherswenson]

In this kind of construction:

source: a_2 is a extend { join_one: b is b extend {
  join_one: c is b on 1 = c.ai
} with astr }

run: a_2 -> {
  nest: x is {
    group_by: ai is b.c.ai
  }
}

I was computing the wrong join usage (which is used for the composite sources feature). Specifically, it was making it out that ai is b.c.ai used joins b.c as well as c (unqualified). Then when the nest was evaluated in the outer query, it tried to figure out what c was (unqualified), and that doesn't exist.