moq-dir: s/group/subgroup/g

This may not be what we really want yet, but it seems to match what was
done on the moq-pub side and it builds, so that's a start...

moq-dir/src/listing.rs

@@ -3,13 +3,13 @@ use bytes::BytesMut;
 use std::collections::{HashSet, VecDeque};
 
 use moq_transport::serve::{
-	GroupReader, GroupWriter, GroupsReader, GroupsWriter, ServeError, TrackReader, TrackReaderMode, TrackWriter,
+	SubgroupReader, SubgroupWriter, SubgroupsReader, SubgroupsWriter, ServeError, TrackReader, TrackReaderMode, TrackWriter,
 };
 
 pub struct ListingWriter {
 	track: Option<TrackWriter>,
-	groups: Option<GroupsWriter>,
-	group: Option<GroupWriter>,
+	groups: Option<SubgroupsWriter>,
+	group: Option<SubgroupWriter>,
 
 	current: HashSet<String>,
 }
@@ -60,7 +60,7 @@ impl ListingWriter {
 		Ok(())
 	}
 
-	fn snapshot(&mut self) -> Result<GroupWriter, ServeError> {
+	fn snapshot(&mut self) -> Result<SubgroupWriter, ServeError> {
 		let mut groups = match self.groups.take() {
 			Some(groups) => groups,
 			None => self.track.take().unwrap().groups()?,
@@ -101,8 +101,8 @@ pub struct ListingReader {
 	track: TrackReader,
 
 	// Keep track of the current group.
-	groups: Option<GroupsReader>,
-	group: Option<GroupReader>,
+	groups: Option<SubgroupsReader>,
+	group: Option<SubgroupReader>,
 
 	// The current state of the listing.
 	current: HashSet<String>,
@@ -130,7 +130,7 @@ impl ListingReader {
 
 		if self.groups.is_none() {
 			self.groups = match self.track.mode().await? {
-				TrackReaderMode::Groups(groups) => Some(groups),
+				TrackReaderMode::Subgroups(groups) => Some(groups),
 				_ => anyhow::bail!("expected groups mode"),
 			};
 		};