Migrate `pallet-atomic-swap` to umbrella crate (#6601)
Part of https://github.com/paritytech/polkadot-sdk/issues/6504
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Co-authored-by:
Giuseppe Re <giuseppe.re@parity.io>
Part of https://github.com/paritytech/polkadot-sdk/issues/6504
---------
Co-authored-by:
Giuseppe Re <giuseppe.re@parity.io>