Revert #2421, rename Raw[Recording]CommandBufer again and move them

[marc0246]

This partially reverts #2421; see #2576 for rationale. Since the plan is that AutoCommandBufferBuilder and co. are eventually removed entirely, we're defensively renaming UnsafeCommandBuffer[Builder] again and removing the command_buffer::sys module such that we don't break these again.

Changelog:
Remove these lines:

### Breaking changes
Changes to command buffers:
- Renamed `AutoCommandBufferBuilder` to `RecordingCommandBuffer` and `AutoCommandBufferBuilder::build` to `end`.
- Renamed `UnsafeCommandBufferBuilder` to `RawRecordingCommandBuffer` and `UnsafeCommandBufferBuilder::build` to `end`.
- Renamed `UnsafeCommandBuffer` to `RawCommandBuffer`.
- `RecordingCommandBuffer` and `RawRecordingCommandBuffer` now take an `Arc<dyn CommandBufferAllocator>` on construction.
- `RecordingCommandBuffer`, `PrimaryAutoCommandBuffer`, `SecondaryAutoCommandBuffer`, `RawRecordingCommandBuffer` and `RawCommandBuffer` no longer have a type parameter for the type of allocator.
- `RawRecordingCommandBuffer::execute_commands` now takes `&RawCommandBuffer`s as argument.

Add these lines:

### Breaking changes
Changes to command buffers:
- Renamed `UnsafeCommandBufferBuilder` to `RecordingCommandBuffer` and `UnsafeCommandBufferBuilder::build` to `end`.
- Renamed `UnsafeCommandBuffer` to `CommandBuffer`.
- `RecordingCommandBuffer` and `CommandBuffer` were moved to the `command_buffer` module; the `command_buffer::sys` module was removed.
- `AutoCommandBufferBuilder`, `PrimaryAutoCommandBuffer`, `SecondaryAutoCommandBuffer`, `RecordingCommandBuffer` and `CommandBuffer` no longer have a type parameter for the type of allocator.
- `RecordingCommandBuffer::execute_commands` now takes `&CommandBuffer`s as argument.