cmd/go: avoid infinite loop in go list -json -e on import cycle

Don't chase import cycles forever preparing list JSON.

Fixes #24086.

Change-Id: Ia1139d0c8d813d068c367a8baee59d240a545617
Reviewed-on: https://go-review.googlesource.com/108016
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/go_test.go

@@ -1373,6 +1373,9 @@ func TestImportCycle(t *testing.T) {
 	if count > 1 {
 		t.Fatal("go build mentioned import cycle more than once")
 	}
+
+	// Don't hang forever.
+	tg.run("list", "-e", "-json", "selfimport")
 }
 
 // cmd/go: custom import path checking should not apply to Go packages without import comment.

src/cmd/go/internal/work/action.go

@@ -339,8 +339,10 @@ func (b *Builder) CompileAction(mode, depMode BuildMode, p *load.Package) *Actio
 			Objdir:  b.NewObjdir(),
 		}
 
-		for _, p1 := range p.Internal.Imports {
-			a.Deps = append(a.Deps, b.CompileAction(depMode, depMode, p1))
+		if p.Error == nil || !p.Error.IsImportCycle {
+			for _, p1 := range p.Internal.Imports {
+				a.Deps = append(a.Deps, b.CompileAction(depMode, depMode, p1))
+			}
 		}
 
 		if p.Standard {