go_1_16: init at 1.16

Changes are minor - I ended up just patching the ssl certs at the root
file, rather than trying to keep up with the various darwin changes.

The externalnetwork test helper location changed, to so I had to update
that patch as well.

- Add xcbuild as propagatedBuildInput on darwin https://github.com/golang/go/commit/7e25bdba5e79d223566745fca2410f725a6dedda

pkgs/development/compilers/go/ssl-cert-file-1.16.patch

@@ -0,0 +1,34 @@
+diff --git a/src/crypto/x509/root.go b/src/crypto/x509/root.go
+index ac92915128..fb1d70c735 100644
+--- a/src/crypto/x509/root.go
++++ b/src/crypto/x509/root.go
+@@ -6,7 +6,11 @@ package x509
+ 
+ //go:generate go run root_ios_gen.go -version 55161.140.3
+ 
+-import "sync"
++import (
++	"io/ioutil"
++	"os"
++	"sync"
++)
+ 
+ var (
+ 	once           sync.Once
+@@ -20,6 +24,16 @@ func systemRootsPool() *CertPool {
+ }
+ 
+ func initSystemRoots() {
++	if file := os.Getenv("NIX_SSL_CERT_FILE"); file != "" {
++		data, err := ioutil.ReadFile(file)
++		if err == nil {
++			roots := NewCertPool()
++			roots.AppendCertsFromPEM(data)
++			systemRoots = roots
++			return
++		}
++	}
++
+ 	systemRoots, systemRootsErr = loadSystemRoots()
+ 	if systemRootsErr != nil {
+ 		systemRoots = nil