cmd/api: reduce parallel 'go list' invocations to a constant
'go list' has its own internal parallelism, so invoking in in parallel
can produce up to quadratic peak memory usage.
Running 'go list' is also very I/O-intensive, so the higher
parallelism does substantially improve latency; unfortunately, we lack
a good way to balance latency against memory footprint, so we need to
sacrifice some latency for reliability.