godoc: sort list of "other packages"

Fixes #2786.

R=r, bradfitz
CC=golang-dev
https://golang.org/cl/5581050

src/cmd/godoc/godoc.go

@@ -1065,6 +1065,7 @@ func (h *httpHandler) getPageInfo(abspath, relpath, pkgname string, mode PageInf
 			}
 		}
 		plist = plist[0:i]
+		sort.Strings(plist)
 	}
 
 	// get examples from *_test.go files