internal/pkg: rename archive checksum helpers

These names are more consistent with other helper names, so rename them while the API is still internal.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/clean_test.go

@@ -283,8 +283,8 @@ func TestClean(t *testing.T) {
 
 			want := tc.want.hash()
 			var checksum pkg.Checksum
-			if err = pkg.HashDir(&checksum, base); err != nil {
-				t.Fatalf("HashDir: error = %v", err)
+			if err = pkg.SumDir(&checksum, base); err != nil {
+				t.Fatalf("SumDir: error = %v", err)
 			} else if checksum != want {
 				t.Error(expectsFrom(base.String()))
 			}