cmd/go: be clear that import loops are bad

There was mail on golang-nuts a few weeks ago
from someone who understood the message perfectly
and knew he had a cyclic dependency but assumed
that Go, like Python or Java, was supposed to handle it.

R=golang-dev, bradfitz, dave
CC=golang-dev
https://golang.org/cl/6488069

src/cmd/go/pkg.go

@@ -250,7 +250,7 @@ func reusePackage(p *Package, stk *importStack) *Package {
 		if p.Error == nil {
 			p.Error = &PackageError{
 				ImportStack: stk.copy(),
-				Err:         "import loop",
+				Err:         "import cycle not allowed",
 			}
 		}
 		p.Incomplete = true