encoding/xml: move unexported const out of exported const block

CL 58210 introduced this constant for reasons I don't understand.
It should not be in the exported const block, which will pollute
godoc output with a "... unexported" notice.

Also since we already have a constant named xmlnsPrefix for "xmlns",
it is very confusing to also have xmlNamespacePrefix for "xml".
If we must have the constant at all, rename it to xmlPrefix.

Change-Id: I15f937454d730005816fcd32b1acca703acf1e51
Reviewed-on: https://go-review.googlesource.com/78121
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/xml/marshal.go

@@ -20,7 +20,6 @@ const (
 	// This is not automatically added to any output of this package,
 	// it is provided as a convenience.
 	Header = `<?xml version="1.0" encoding="UTF-8"?>` + "\n"
-	xmlNamespacePrefix = "xml"
 )
 
 // Marshal returns the XML encoding of v.
@@ -324,7 +323,7 @@ func (p *printer) createAttrPrefix(url string) string {
 	// (The "http://www.w3.org/2000/xmlns/" name space is also predefined as "xmlns",
 	// but users should not be trying to use that one directly - that's our job.)
 	if url == xmlURL {
-		return xmlNamespacePrefix
+		return xmlPrefix
 	}
 
 	// Need to define a new name space.

src/encoding/xml/xml.go

@@ -338,6 +338,7 @@ func (d *Decoder) Token() (Token, error) {
 const (
 	xmlURL      = "http://www.w3.org/XML/1998/namespace"
 	xmlnsPrefix = "xmlns"
+	xmlPrefix   = "xml"
 )
 
 // Apply name space translation to name n.
@@ -349,7 +350,7 @@ func (d *Decoder) translate(n *Name, isElementName bool) {
 		return
 	case n.Space == "" && !isElementName:
 		return
-	case n.Space == xmlNamespacePrefix:
+	case n.Space == xmlPrefix:
 		n.Space = xmlURL
 	case n.Space == "" && n.Local == xmlnsPrefix:
 		return