switch our import prefixes to be unique per file, not per generation request

[devoncarew]

Currently our import prefixes ($0, $1`, ...) are unique to a generation request; we monotonically increment the prefix based on the total number of prefixed imports in a generation request. We should instead switch to having the prefix being unique to that file. In addition, we should make sure the generation is stable (we always get the same set of prefixes given the same request info).

See #952 as a related issue.