imports: fix circular imports

goimports will add an import for the package of target source file accidentally,
so check if package path is different from target source file when finding import candidates.

Fixes golang/go#30663

Change-Id: I77c29bc74bef6c888e63ccb501b013a5fbc30b5c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/170238
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: kazyshr

imports/fix.go

@@ -1043,7 +1043,7 @@ func findImport(ctx context.Context, env *fixEnv, dirScan []*pkg, pkgName string
 	// Find candidate packages, looking only at their directory names first.
 	var candidates []pkgDistance
 	for _, pkg := range dirScan {
-		if pkgIsCandidate(filename, pkgName, pkg) {
+		if pkg.dir != pkgDir && pkgIsCandidate(filename, pkgName, pkg) {
 			candidates = append(candidates, pkgDistance{
 				pkg:      pkg,
 				distance: distance(pkgDir, pkg.dir),

imports/fix_test.go

@@ -2066,6 +2066,28 @@ var _ = fmt.Printf
 
 }
 
+// Tests that an input file's own package is ignored.
+func TestIgnoreOwnPackage(t *testing.T) {
+	const input = `package pkg
+
+const _ = pkg.X
+`
+	const want = `package pkg
+
+const _ = pkg.X
+`
+
+	testConfig{
+		module: packagestest.Module{
+			Name: "foo.com",
+			Files: fm{
+				"pkg/a.go": "package pkg\nconst X = 1",
+				"pkg/b.go": input,
+			},
+		},
+	}.processTest(t, "foo.com", "pkg/b.go", nil, nil, want)
+}
+
 func TestPkgIsCandidate(t *testing.T) {
 	tests := []struct {
 		name     string