move func PartStructure from webhook to queue, so it isn't tracked anymore for apidiff changes

the types in webhook should be subjected to apidiff'ing, this was a shared
function. it is better off in package queue. also change the apidiff script so
it leaves apidiff/next.txt empty when there aren't any changes. makes it easier
to rotate the files after releases where nothing changed (a common occurrence).

webhook/webhook.go

@@ -8,12 +8,7 @@
 package webhook
 
 import (
-	"errors"
-	"strings"
 	"time"
-
-	"github.com/mjl-/mox/message"
-	"github.com/mjl-/mox/mlog"
 )
 
 // OutgoingEvent is an activity for an outgoing delivery. Either generated by the
@@ -145,35 +140,3 @@ type Structure struct {
 	DecodedSize        int64             // Size of content after decoding content-transfer-encoding. For text and HTML parts, this can be larger than the data returned since this size includes \r\n line endings.
 	Parts              []Structure       // Subparts of a multipart message, possibly recursive.
 }
-
-// PartStructure returns a Structure for a parsed message part.
-func PartStructure(log mlog.Log, p *message.Part) (Structure, error) {
-	parts := make([]Structure, len(p.Parts))
-	for i := range p.Parts {
-		var err error
-		parts[i], err = PartStructure(log, &p.Parts[i])
-		if err != nil && !errors.Is(err, message.ErrParamEncoding) {
-			return Structure{}, err
-		}
-	}
-	disp, filename, err := p.DispositionFilename()
-	if err != nil && errors.Is(err, message.ErrParamEncoding) {
-		log.Debugx("parsing disposition/filename", err)
-	} else if err != nil {
-		return Structure{}, err
-	}
-	s := Structure{
-		ContentType:        strings.ToLower(p.MediaType + "/" + p.MediaSubType),
-		ContentTypeParams:  p.ContentTypeParams,
-		ContentID:          p.ContentID,
-		ContentDisposition: strings.ToLower(disp),
-		Filename:           filename,
-		DecodedSize:        p.DecodedSize,
-		Parts:              parts,
-	}
-	// Replace nil map with empty map, for easier to use JSON.
-	if s.ContentTypeParams == nil {
-		s.ContentTypeParams = map[string]string{}
-	}
-	return s, nil
-}