refactor: Relocate message handler interface

contracts/SpokePool.sol

@@ -3,6 +3,7 @@ pragma solidity ^0.8.0;
 
 import "./MerkleLib.sol";
 import "./external/interfaces/WETH9Interface.sol";
+import "./interfaces/SpokePoolMessageHandler.sol";
 import "./interfaces/SpokePoolInterface.sol";
 import "./interfaces/V3SpokePoolInterface.sol";
 import "./upgradeable/MultiCallerUpgradeable.sol";
@@ -16,16 +17,6 @@ import "@openzeppelin/contracts-upgradeable/proxy/utils/UUPSUpgradeable.sol";
 import "@openzeppelin/contracts-upgradeable/security/ReentrancyGuardUpgradeable.sol";
 import "@openzeppelin/contracts/utils/math/SignedMath.sol";
 
-// This interface is expected to be implemented by any contract that expects to receive messages from the SpokePool.
-interface AcrossMessageHandler {
-    function handleV3AcrossMessage(
-        address tokenSent,
-        uint256 amount,
-        address relayer,
-        bytes memory message
-    ) external;
-}
-
 /**
  * @title SpokePool
  * @notice Base contract deployed on source and destination chains enabling depositors to transfer assets from source to

contracts/handlers/MulticallHandler.sol

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 pragma solidity ^0.8.0;
 
-import "../SpokePool.sol";
+import "../interfaces/SpokePoolMessageHandler.sol";
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
 import "@openzeppelin/contracts/utils/Address.sol";

contracts/interfaces/SpokePoolMessageHandler.sol

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: BUSL-1.1
+pragma solidity ^0.8.0;
+
+// This interface is expected to be implemented by any contract that expects to receive messages from the SpokePool.
+interface AcrossMessageHandler {
+    function handleV3AcrossMessage(
+        address tokenSent,
+        uint256 amount,
+        address relayer,
+        bytes memory message
+    ) external;
+}