feat(internal): marshal self-closing tags in XML (#1225)

Currently self-closing tags are marshaled as `<a></a>` which is not the
preferred format `<a/>`.

With the current implementation of `encoding/xml`, self-closing tags are
expanded to `StartElement` and `EndElement` so both elements are written
to output.

In this PR, a field `Empty` is added to both elements to indicate
whether the current element is empty. During encoding:
 - `/` is written before `>` for an empty `StartElement`
 - nothing will be written for an empty `EndElement`

Considering that we only want tabs not being escaped, this PR modifies
`escapeNewline` to `escapeWhitespace` to indicate if we want all
whitespace characters escaped.

internal/resolution/manifest/__snapshots__/maven_test.snap

@@ -42,6 +42,7 @@
       <groupId>org.example</groupId>
       <artifactId>abc</artifactId>
       <version>1.0.1</version>
+      <optional/>
     </dependency>
     <dependency>
       <groupId>org.example</groupId>
@@ -186,6 +187,7 @@
       <groupId>org.example</groupId>
       <artifactId>abc</artifactId>
       <version>1.0.2</version>
+      <optional/>
     </dependency>
     <dependency>
       <groupId>org.example</groupId>

internal/resolution/manifest/fixtures/maven/my-app/pom.xml

@@ -40,6 +40,7 @@
       <groupId>org.example</groupId>
       <artifactId>abc</artifactId>
       <version>1.0.1</version>
+      <optional/>
     </dependency>
     <dependency>
       <groupId>org.example</groupId>

internal/thirdparty/xml/marshal.go

@@ -216,11 +216,11 @@ func (enc *Encoder) EncodeToken(t Token) error {
 			return err
 		}
 	case EndElement:
-		if err := p.writeEnd(t.Name); err != nil {
+		if err := p.writeEnd(t.Name, t.Empty); err != nil {
 			return err
 		}
 	case CharData:
-		p.Write(t)
+		escapeText(p, t, false)
 	case Comment:
 		if bytes.Contains(t, endComment) {
 			return fmt.Errorf("xml: EncodeToken of Comment containing --> marker")
@@ -495,6 +495,7 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 	if startTemplate != nil {
 		start.Name = startTemplate.Name
 		start.Attr = append(start.Attr, startTemplate.Attr...)
+		start.Empty = startTemplate.Empty
 	} else if tinfo.xmlname != nil {
 		xmlname := tinfo.xmlname
 		if xmlname.name != "" {
@@ -569,7 +570,7 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 		return err
 	}
 
-	if err := p.writeEnd(start.Name); err != nil {
+	if err := p.writeEnd(start.Name, start.Empty); err != nil {
 		return err
 	}
 
@@ -712,7 +713,7 @@ func (p *printer) marshalTextInterface(val encoding.TextMarshaler, start StartEl
 		return err
 	}
 	EscapeText(p, text)
-	return p.writeEnd(start.Name)
+	return p.writeEnd(start.Name, start.Empty)
 }
 
 // writeStart writes the given start element.
@@ -750,11 +751,16 @@ func (p *printer) writeStart(start *StartElement) error {
 		p.EscapeString(attr.Value)
 		p.WriteByte('"')
 	}
+
+	if start.Empty {
+		p.WriteByte('/')
+	}
+
 	p.WriteByte('>')
 	return nil
 }
 
-func (p *printer) writeEnd(name Name) error {
+func (p *printer) writeEnd(name Name, empty bool) error {
 	if name.Local == "" {
 		return fmt.Errorf("xml: end tag with no name")
 	}
@@ -769,11 +775,13 @@ func (p *printer) writeEnd(name Name) error {
 	}
 	p.tags = p.tags[:len(p.tags)-1]
 
-	p.writeIndent(-1)
-	p.WriteByte('<')
-	p.WriteByte('/')
-	p.WriteString(name.Local)
-	p.WriteByte('>')
+	if !empty {
+		p.writeIndent(-1)
+		p.WriteByte('<')
+		p.WriteByte('/')
+		p.WriteString(name.Local)
+		p.WriteByte('>')
+	}
 	p.popPrefix()
 	return nil
 }
@@ -1087,7 +1095,7 @@ func (s *parentStack) trim(parents []string) error {
 		}
 	}
 	for i := len(s.stack) - 1; i >= split; i-- {
-		if err := s.p.writeEnd(Name{Local: s.stack[i]}); err != nil {
+		if err := s.p.writeEnd(Name{Local: s.stack[i]}, false); err != nil {
 			return err
 		}
 	}