go/doc: use correct escaper for URL

Despite the name, URL escaping is for a small subpiece of the URL only.
This particular URL is being emitted in an <a href=""> tag and in that
context it should be HTML escaped, not URL escaped.

In addition to fixing a bug, this change cuts a dependency chain
from go/doc to net, clearing the way for use of cgo
(which imports go/doc) during the compilation of package net.

R=gri
CC=golang-dev
https://golang.org/cl/1960050

src/pkg/go/doc/comment.go

@@ -8,7 +8,6 @@ package doc
 
 import (
 	"go/ast"
-	"http" // for URLEscape
 	"io"
 	"regexp"
 	"strings"
@@ -227,7 +226,7 @@ func emphasize(w io.Writer, line []byte, words map[string]string, nice bool) {
 		// write match
 		if len(url) > 0 {
 			w.Write(html_a)
-			w.Write([]byte(http.URLEscape(url)))
+			template.HTMLEscape(w, []byte(url))
 			w.Write(html_aq)
 		}
 		if italics {