cmd/go: add dependencies for implicit SWIG imports

This fixes the misc/swig tests.

Change-Id: I60c87bbd361fe8b4f69e4507b25dc99a226da3d7
Reviewed-on: https://go-review.googlesource.com/76610
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -958,13 +958,23 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 
 	// Cgo translation adds imports of "runtime/cgo" and "syscall",
 	// except for certain packages, to avoid circular dependencies.
-	if len(p.CgoFiles) > 0 && (!p.Standard || !cgoExclude[p.ImportPath]) {
+	if p.UsesCgo() && (!p.Standard || !cgoExclude[p.ImportPath]) {
 		addImport("runtime/cgo")
 	}
-	if len(p.CgoFiles) > 0 && (!p.Standard || !cgoSyscallExclude[p.ImportPath]) {
+	if p.UsesCgo() && (!p.Standard || !cgoSyscallExclude[p.ImportPath]) {
 		addImport("syscall")
 	}
 
+	// SWIG adds imports of some standard packages.
+	if p.UsesSwig() {
+		addImport("runtime/cgo")
+		addImport("syscall")
+		addImport("sync")
+
+		// TODO: The .swig and .swigcxx files can use
+		// %go_import directives to import other packages.
+	}
+
 	// The linker loads implicit dependencies.
 	if p.Name == "main" && !p.Internal.ForceLibrary {
 		for _, dep := range LinkerDeps(p) {