Refactor populateItems (#3732)

<!-- PR description-->

Benefits:
* Narrowed scope & better testability. 
* Preps it for follow up PRs which will inject per-item corso extension interfaces
* Not adding unit tests yet, as the args & outputs of this new function will change in a follow up PR.
---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup

#### Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* #<issue>

#### Test Plan

<!-- How will this be tested prior to merging.-->
- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/m365/onedrive/collection.go

@@ -403,17 +403,21 @@ func readItemContents(
 	return rc, nil
 }
 
+type driveStats struct {
+	dirsRead   int64
+	itemsRead  int64
+	byteCount  int64
+	itemsFound int64
+	dirsFound  int64
+}
+
 // populateItems iterates through items added to the collection
 // and uses the collection `itemReader` to read the item
 func (oc *Collection) populateItems(ctx context.Context, errs *fault.Bus) {
 	var (
-		byteCount  int64
-		itemsRead  int64
-		dirsRead   int64
-		itemsFound int64
-		dirsFound  int64
-		wg         sync.WaitGroup
 		el    = errs.Local()
+		stats driveStats
+		wg    sync.WaitGroup
 	)
 
 	// Retrieve the OneDrive folder path to set later in
@@ -445,12 +449,36 @@ func (oc *Collection) populateItems(ctx context.Context, errs *fault.Bus) {
 
 		wg.Add(1)
 
-		go func(ctx context.Context, item models.DriveItemable) {
+		go func(item models.DriveItemable) {
 			defer wg.Done()
 			defer func() { <-semaphoreCh }()
 
 			// Read the item
+			oc.populateDriveItem(
+				ctx,
+				parentPath,
+				item,
+				&stats,
+				errs)
+
+			folderProgress <- struct{}{}
+		}(item)
+	}
+
+	wg.Wait()
+
+	oc.reportAsCompleted(ctx, int(stats.itemsFound), int(stats.itemsRead), stats.byteCount)
+}
+
+func (oc *Collection) populateDriveItem(
+	ctx context.Context,
+	parentPath *path.Builder,
+	item models.DriveItemable,
+	stats *driveStats,
+	errs *fault.Bus,
+) {
 	var (
+		el           = errs.Local()
 		itemID       = ptr.Val(item.GetId())
 		itemName     = ptr.Val(item.GetName())
 		itemSize     = ptr.Val(item.GetSize())
@@ -473,12 +501,12 @@ func (oc *Collection) populateItems(ctx context.Context, errs *fault.Bus) {
 	isFile := item.GetFile() != nil
 
 	if isFile {
-				atomic.AddInt64(&itemsFound, 1)
+		atomic.AddInt64(&stats.itemsFound, 1)
 
 		metaFileName = itemID
 		metaSuffix = metadata.MetaFileSuffix
 	} else {
-				atomic.AddInt64(&dirsFound, 1)
+		atomic.AddInt64(&stats.dirsFound, 1)
 
 		// metaFileName not set for directories so we get just ".dirmeta"
 		metaSuffix = metadata.DirMetaFileSuffix
@@ -546,21 +574,12 @@ func (oc *Collection) populateItems(ctx context.Context, errs *fault.Bus) {
 
 	// Item read successfully, add to collection
 	if isFile {
-				atomic.AddInt64(&itemsRead, 1)
+		atomic.AddInt64(&stats.itemsRead, 1)
 	} else {
-				atomic.AddInt64(&dirsRead, 1)
+		atomic.AddInt64(&stats.dirsRead, 1)
 	}
 
-			// byteCount iteration
-			atomic.AddInt64(&byteCount, itemSize)
-
-			folderProgress <- struct{}{}
-		}(ctx, item)
-	}
-
-	wg.Wait()
-
-	oc.reportAsCompleted(ctx, int(itemsFound), int(itemsRead), byteCount)
+	atomic.AddInt64(&stats.byteCount, itemSize)
 }
 
 func (oc *Collection) reportAsCompleted(ctx context.Context, itemsFound, itemsRead int, byteCount int64) {