Change dependency source to work around Truffle limitation (OpenZeppelin#3218)

Author: JulissaDantes

contracts/interfaces/IERC2981.sol

@@ -3,7 +3,7 @@
 
 pragma solidity ^0.8.0;
 
-import "./IERC165.sol";
+import "../utils/introspection/IERC165.sol";
 
 /**
  * @dev Interface for the NFT Royalty Standard.

contracts/token/ERC20/extensions/ERC20FlashMint.sol

@@ -3,7 +3,8 @@
 
 pragma solidity ^0.8.0;
 
-import "../../../interfaces/IERC3156.sol";
+import "../../../interfaces/IERC3156FlashBorrower.sol";
+import "../../../interfaces/IERC3156FlashLender.sol";
 import "../ERC20.sol";
 
 /**