cmd/go/internal/modconv: put old module path in require section

This change modifies Go to generate correct go.mod file.
Currently go-mod puts wrong module path in requires when
Gopkg.toml contains source config. for example, there is
a constraint declaration in Gopkg.toml as following:

[[constraint]]
  name = "github.com/minio/minio-go"
  revision = "0415038e6086735b8c917d7fdb2ca93f07d7fb60"
  source = "https://github.com/liuchang0812/minio-go"

go-mod puts `github.com/liuchang0812/minio-go` in requires.
but, it should be `github.com/minio/minio-go`

Fixes: #33406

src/cmd/go/internal/modconv/convert.go

@@ -91,8 +91,10 @@ func ConvertLegacyConfig(f *modfile.File, file string, data []byte) error {
 			if err != nil {
 				return fmt.Errorf("add replace: %v", err)
 			}
+			f.AddNewRequire(re.Old.Path, need[path], false)
+		} else {
+			f.AddNewRequire(path, need[path], false)
 		}
-		f.AddNewRequire(path, need[path], false)
 	}
 
 	f.Cleanup()

src/cmd/go/internal/modconv/convert_test.go

@@ -145,6 +145,27 @@ func TestConvertLegacyConfig(t *testing.T) {
 				google.golang.org/grpc v1.10.0
 			)`,
 		},
+
+		{
+			// testes replace
+			"github.com/liuchang0812/supreme-octo-memory", "v6.0.10+incompatible",
+			`module github.com/liuchang0812/supreme-octo-memory
+
+			 require (
+        			 github.com/dustin/go-humanize v1.0.0
+        			 github.com/go-ini/ini v1.44.0
+        			 github.com/konsorten/go-windows-terminal-sequences v1.0.2
+        			 github.com/minio/minio-go v6.0.10+incompatible
+        			 github.com/mitchellh/go-homedir v1.1.0
+        			 github.com/sirupsen/logrus v1.4.2
+        			 golang.org/x/crypto v0.0.0-20190701094942-4def268fd1a4
+        			 golang.org/x/net v0.0.0-20190724013045-ca1201d0de80
+        			 golang.org/x/sys v0.0.0-20190801041406-cbf593c0f2f3
+        			 golang.org/x/text v0.3.2
+			)
+
+			replace github.com/minio/minio-go v6.0.10+incompatible => github.com/liuchang0812/minio-go v6.0.10+incompatible`,
+		},
 	}
 
 	for _, tt := range tests {