cmd/go: print each import error only once

This change prevents import errors from being printed multiple times.
Creating a bare-bones package 'p' with only one file importing itself
and running 'go build p', the current implementation gives this error
message:

	can't load package: import cycle not allowed
	package p
		imports p
	import cycle not allowed
	package p
		imports p

With this change we will show the message only once.

Updates #23295

Change-Id: I653b34c1c06c279f3df514f12ec0b89745a7e64a
Reviewed-on: https://go-review.googlesource.com/86535Reviewed-by: Harald Nordgren <haraldnordgren@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -1359,6 +1359,22 @@ func TestImportCommentConflict(t *testing.T) {
 	tg.grepStderr("found import comments", "go build did not mention comment conflict")
 }
 
+func TestImportCycle(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata/importcycle"))
+	tg.runFail("build", "selfimport")
+
+	count := tg.grepCountBoth("import cycle not allowed")
+	if count == 0 {
+		t.Fatal("go build did not mention cyclical import")
+	}
+	if count > 1 {
+		t.Fatal("go build mentioned import cycle more than once")
+	}
+}
+
 // cmd/go: custom import path checking should not apply to Go packages without import comment.
 func TestIssue10952(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)

src/cmd/go/internal/load/pkg.go

@@ -1473,6 +1473,7 @@ func PackagesForBuild(args []string) []*Package {
 	for _, pkg := range pkgs {
 		if pkg.Error != nil {
 			base.Errorf("can't load package: %s", pkg.Error)
+			printed[pkg.Error] = true
 		}
 		for _, err := range pkg.DepsErrors {
 			// Since these are errors in dependencies,

src/cmd/go/testdata/importcycle/src/selfimport/selfimport.go

+package selfimport
+
+import "selfimport"