deps: allow crypto/x509 cgo and io/ioutil.

In order to land 5700087 (which moves the knowledge of how to get the
root certificates for the system from crypto/tls to crypto/x509), we
need to relax the restrictions on crypto/x509. Afterwards, we can
probably tighten them up in crypto/tls.

R=golang-dev, rsc, krautz
CC=golang-dev
https://golang.org/cl/5753060

src/pkg/go/build/deps_test.go

@@ -293,7 +293,7 @@ var pkgDeps = map[string][]string{
 		"L3", "CRYPTO-MATH", "CGO", "OS",
 		"crypto/x509", "encoding/pem", "net", "syscall",
 	},
-	"crypto/x509":      {"L3", "CRYPTO-MATH", "crypto/x509/pkix", "encoding/pem"},
+	"crypto/x509":      {"L3", "CRYPTO-MATH", "OS", "CGO", "crypto/x509/pkix", "encoding/pem"},
 	"crypto/x509/pkix": {"L3", "CRYPTO-MATH"},
 
 	// Simple net+crypto-aware packages.