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/reply.go

@@ -10,6 +10,7 @@ import (
 
 	"git.sr.ht/~sircmpwn/getopt"
 
+	"git.sr.ht/~sircmpwn/aerc/lib"
 	"git.sr.ht/~sircmpwn/aerc/models"
 	"git.sr.ht/~sircmpwn/aerc/widgets"
 )
@@ -169,14 +170,12 @@ func (reply) Execute(aerc *widgets.Aerc, args []string) error {
 			template = aerc.Config().Templates.QuotedReply
 		}
 
-		part := findPlaintext(msg.BodyStructure, nil)
+		part := lib.FindPlaintext(msg.BodyStructure, nil)
 		if part == nil {
-			//mkey... let's get the first thing that isn't a container
-			part = findFirstNonMultipart(msg.BodyStructure, nil)
-			if part == nil {
-				// give up, use whatever is first
-				part = []int{1}
-			}
+			// mkey... let's get the first thing that isn't a container
+			// if that's still nil it's either not a multipart msg (ok) or
+			// broken (containers only)
+			part = lib.FindFirstNonMultipart(msg.BodyStructure, nil)
 		}
 		store.FetchBodyPart(msg.Uid, part, func(reader io.Reader) {
 			buf := new(bytes.Buffer)