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/archive_test.go

@@ -141,10 +141,10 @@ func TestArchive(t *testing.T) {
 				t.Parallel()
 
 				var got pkg.Checksum
-				if err := pkg.HashFS(&got, tc.fsys, "."); err != nil {
-					t.Fatalf("HashFS: error = %v", err)
+				if err := pkg.SumFS(&got, tc.fsys, "."); err != nil {
+					t.Fatalf("SumFS: error = %v", err)
 				} else if got != tc.sum {
-					t.Fatalf("HashFS: %v", &pkg.ChecksumMismatchError{
+					t.Fatalf("SumFS: %v", &pkg.ChecksumMismatchError{
 						Got:  got,
 						Want: tc.sum,
 					})