godoc: canonicalize codewalk paths

R=rsc
CC=golang-dev
https://golang.org/cl/1729046

lib/godoc/codewalk.html

@@ -15,7 +15,7 @@
     <div id="code-area">
       <div id="code-header" align="center">
         <a id="code-popout-link" href="" target="_blank">
-          <img title="View code in new window" alt="Pop Out Code" src="popout.png" style="display: block; float: right;"/>
+          <img title="View code in new window" alt="Pop Out Code" src="/doc/codewalk/popout.png" style="display: block; float: right;"/>
         </a>
         <select id="code-selector">
           {.repeated section File}

src/cmd/godoc/codewalk.go

@@ -63,6 +63,11 @@ func codewalk(c *http.Conn, r *http.Request) {
 		return
 	}
 
+	// Canonicalize the path and redirect if changed
+	if redirect(c, r) {
+		return
+	}
+
 	b := applyTemplate(codewalkHTML, "codewalk", cw)
 	servePage(c, "Codewalk: "+cw.Title, "", "", b)
 }