block: qcow: Flatten internal and worker modules

Remove the internal and worker submodule layers from the QCOW2
format directory. The former internal files become direct children
of qcow, with internal/mod.rs turning into parser.rs. The worker
backends move up as engine_sync.rs and engine_uring.rs, and
worker/mod.rs, which held only module declarations, is dropped.

The public parser types are now surfaced at the qcow module level,
so external callers use block::formats::qcow instead of reaching
into the internal module.

Assisted-by: Claude:Opus-4.8
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/lib.rs

@@ -28,7 +28,7 @@ use std::str::FromStr;
 use std::{cmp, io, mem, result};
 
 pub use aligned_file::AlignedFile;
-use formats::qcow::internal as qcow;
+use formats::qcow;
 #[cfg(feature = "io_uring")]
 use io_uring::{IoUring, Probe, opcode};
 use libc::{