cmd/go: expand 'internal' rule to all repositories

See golang.org/s/go14internal.

Fixes #10479.

Change-Id: I8c657dadeb5f10af060f22dedc15c1af989d4519
Reviewed-on: https://go-review.googlesource.com/9156Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/pkg.go

@@ -322,13 +322,6 @@ func disallowInternal(srcDir string, p *Package, stk *importStack) *Package {
 	// An import of a path containing the element “internal”
 	// is disallowed if the importing code is outside the tree
 	// rooted at the parent of the “internal” directory.
-	//
-	// ... For Go 1.4, we will implement the rule first for $GOROOT, but not $GOPATH.
-
-	// Only applies to $GOROOT.
-	if !p.Standard {
-		return p
-	}
 
 	// The stack includes p.ImportPath.
 	// If that's the only thing on the stack, we started

src/cmd/go/test.bash

@@ -156,9 +156,13 @@ elif ! grep 'use of internal package not allowed' testdata/std.out >/dev/null; t
 	ok=false
 fi
 
-TEST 'internal packages outside $GOROOT are not respected'
-if ! ./testgo build -v ./testdata/testinternal2; then
-	echo "go build ./testdata/testinternal2 failed"
+TEST 'internal packages outside $GOROOT are respected (as of Go 1.5)'
+if ./testgo build -v ./testdata/testinternal2 >testdata/std.out 2>&1; then
+	echo "go build ./testdata/testinternal2 succeeded incorrectly"
+	ok=false
+elif ! grep 'use of internal package not allowed' testdata/std.out >/dev/null; then
+	echo "wrong error message for testdata/testinternal2"
+	cat std.out
 	ok=false
 fi