topdown: only amend comprehensions with used bindings (#3608)

Before, all the available bindings had been added to the comprehension in

    package pkg
    p[x] { q[x] }
    q[x] {
            y := {1 | input} # <---
            x := "foo"
    }

So after partial eval, the support rule was

    p["foo"] {
            __local0__2 = {1 |
                    input
                    __local1__2 = x12
            }
    }

Now, only variables that are used in the comprehension in question are
added. The partial eval result becomes

    package partial.pkg

    p["foo"] {
            __local0__2 = {1 | input}
    }

An extra round of the process was added when both lhs and rhs are
comprehensions.

Fixes #3557.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>