Create and use generic tombstone collection (#4339)

Incremental backups requires us to mark some
folders as deleted by creating a collection
with state `data.DeletedState`

This PR creates a simple, generic "tombstone"
collection that does just that

The PR additionally uses the tombstone
collection in place of more complicated
implementations where any easy switch is
possible. Deleted collections in OneDrive
require more work since tests attempt to
cast to a concrete type

---

#### 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

- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup

#### Issue(s)

* #4191

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [x] 💚 E2E

src/internal/data/collection_test.go

@@ -153,3 +153,17 @@ func (suite *CollectionSuite) TestNewBaseCollection() {
 		})
 	}
 }
+
+func (suite *CollectionSuite) TestNewTombstoneCollection() {
+	t := suite.T()
+
+	fooP, err := path.Build("t", "u", path.ExchangeService, path.EmailCategory, false, "foo")
+	require.NoError(t, err, clues.ToCore(err))
+
+	c := NewTombstoneCollection(fooP, control.Options{})
+	assert.Nil(t, c.FullPath(), "full path")
+	assert.Equal(t, fooP, c.PreviousPath(), "previous path")
+	assert.Nil(t, c.LocationPath(), "location path")
+	assert.Equal(t, DeletedState, c.State(), "state")
+	assert.False(t, c.DoNotMergeItems(), "do not merge")
+}

src/internal/data/implementations.go

@@ -6,6 +6,7 @@ import (
 	"github.com/alcionai/clues"
 
 	"github.com/alcionai/corso/src/pkg/control"
+	"github.com/alcionai/corso/src/pkg/fault"
 	"github.com/alcionai/corso/src/pkg/path"
 )
 
@@ -136,3 +137,30 @@ func (col BaseCollection) DoNotMergeItems() bool {
 func (col BaseCollection) Opts() control.Options {
 	return col.opts
 }
+
+// -----------------------------------------------------------------------------
+// tombstoneCollection
+// -----------------------------------------------------------------------------
+
+func NewTombstoneCollection(
+	prev path.Path,
+	opts control.Options,
+) *tombstoneCollection {
+	return &tombstoneCollection{
+		BaseCollection: NewBaseCollection(nil, prev, nil, opts, false),
+	}
+}
+
+// tombstoneCollection is a collection that marks a folder (and folders under it
+// if they aren't explicitly noted in other collecteds) as deleted. It doesn't
+// contain any items.
+type tombstoneCollection struct {
+	BaseCollection
+}
+
+// Items never returns any data for tombstone collections because the collection
+// only denotes the deletion of the current folder and possibly subfolders. All
+// items contained in the deleted folder are also deleted.
+func (col *tombstoneCollection) Items(context.Context, *fault.Bus) <-chan Item {
+	return nil
+}

src/internal/m365/collection/exchange/backup.go

@@ -241,20 +241,7 @@ func populateCollections(
 			continue
 		}
 
-		edc := NewCollection(
+		collections[id] = data.NewTombstoneCollection(prevPath, ctrlOpts)
-			data.NewBaseCollection(
-				nil, // marks the collection as deleted
-				prevPath,
-				nil, // tombstones don't need a location
-				ctrlOpts,
-				false),
-			qp.ProtectedResource.ID(),
-			bh.itemHandler(),
-			nil,
-			nil,
-			false,
-			statusUpdater)
-		collections[id] = edc
 	}
 
 	logger.Ctx(ctx).Infow(

src/internal/m365/collection/groups/backup.go

@@ -233,20 +233,7 @@ func populateCollections(
 			continue
 		}
 
-		edc := NewCollection(
+		collections[id] = data.NewTombstoneCollection(prevPath, ctrlOpts)
-			data.NewBaseCollection(
-				nil, // marks the collection as deleted
-				prevPath,
-				nil, // tombstones don't need a location
-				ctrlOpts,
-				false),
-			bh,
-			qp.ProtectedResource.ID(),
-			nil, // no items added
-			nil, // this deletes a directory, so no items deleted either
-			statusUpdater)
-
-		collections[id] = &edc
 	}
 
 	logger.Ctx(ctx).Infow(