cue: prepare to hoist builtin code

Staightforward renaming and deletion of dead code.

Change-Id: I3b77b0b0f50914791cc00653a44c82dfebd0b0db
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6881
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl