Remove hard coded bodystruct path everywhere

Aerc usually used the path []int{1} if it didn't know what the proper path is.
However this only works for multipart messages and breaks if it isn't one.

This patch removes all the hard coding and extracts the necessary helpers to lib.

commands/msg/forward.go

@@ -10,6 +10,7 @@ import (
 	"path"
 	"strings"
 
+	"git.sr.ht/~sircmpwn/aerc/lib"
 	"git.sr.ht/~sircmpwn/aerc/models"
 	"git.sr.ht/~sircmpwn/aerc/widgets"
 	"git.sr.ht/~sircmpwn/aerc/worker/types"
@@ -137,12 +138,10 @@ func (forward) Execute(aerc *widgets.Aerc, args []string) error {
 		}
 
 		// TODO: add attachments!
-		part := findPlaintext(msg.BodyStructure, nil)
+		part := lib.FindPlaintext(msg.BodyStructure, nil)
 		if part == nil {
-			part = findFirstNonMultipart(msg.BodyStructure, nil)
-			if part == nil {
-				part = []int{1}
-			}
+			part = lib.FindFirstNonMultipart(msg.BodyStructure, nil)
+			// if it's still nil here, we don't have a multipart msg, that's fine
 		}
 		store.FetchBodyPart(msg.Uid, part, func(reader io.Reader) {
 			buf := new(bytes.Buffer)