cmd/api: run half as many go list calls in parallel

We currently run one 'go list' invocation per GOMAXPROC. Since the go
command uses memory and has its own internal parallelism, that's
unlikely to be an efficient use of resources. Run half as many. I
suspect that's still too many but this should fix our OOMs.

For #49957.

Change-Id: Id06b6e0f0d96387a2a050e400f38bde6ba71aa60
Reviewed-on: https://go-review.googlesource.com/c/go/+/370376
Trust: Heschi Kreinick <[email protected]>
Run-TryBot: Heschi Kreinick <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: heschi

src/cmd/api/goapi.go

@@ -460,7 +460,7 @@ type listImports struct {
 var listCache sync.Map // map[string]listImports, keyed by contextName
 
 // listSem is a semaphore restricting concurrent invocations of 'go list'.
-var listSem = make(chan semToken, runtime.GOMAXPROCS(0))
+var listSem = make(chan semToken, ((runtime.GOMAXPROCS(0)-1)/2)+1)
 
 type semToken struct{}