[dev.go2go] go/go2go: don't import renamed package twice

We keep an import spec if it uses a local alias. Avoid importing it
again, unless it is required by something else.

Fixes #40318

Change-Id: If90de3bf30412645c9144083372e6b07df3c6a0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/244621
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/go2go/rewrite.go

@@ -284,11 +284,16 @@ func rewriteAST(fset *token.FileSet, importer *Importer, importPath string, tpkg
 			}
 			// We picked up Go 2 imports above, but we still
 			// need to pick up Go 1 imports here.
-			path := strings.TrimPrefix(strings.TrimSuffix(imp.Path.Value, `"`), `"`)
-			if imps[path] {
+			path, err := strconv.Unquote(imp.Path.Value)
+			if err != nil || imps[path] {
 				continue
 			}
-			imps[path] = true
+			if imp.Name == nil {
+				// If Name != nil we are keeping the spec.
+				// Don't add the import again here,
+				// only if it is needed elsewhere.
+				imps[path] = true
+			}
 			for _, p := range importer.transitiveImports(path) {
 				imps[p] = true
 			}

test/gen/g042.go2

@@ -0,0 +1,18 @@
+// compile
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 40318
+package p
+
+import (
+	crand "crypto/rand"
+	"math/rand"
+)
+
+func F() {
+	_ = crand.Reader
+	_ = rand.Source(nil)
+}