N-06 Explicit imports

[DanielVF]

OZ Says

The use of non-explicit imports in the codebase can decrease code clarity and may create naming conflicts between locally defined and imported variables. This is particularly relevant when multiple contracts exist within the same Solidity file or when inheritance chains are long.

Throughout the codebase, global imports are being used:

• The import "OpenZeppelin/[email protected]/contracts/token/ERC20/IERC20.sol"; import in FixedRateRewardsSource.sol
• The import "OpenZeppelin/[email protected]/contracts/token/ERC20/extensions/ERC20Burnable.sol"; import in Migrator.sol
• The import "./Governable.sol"; import in Migrator.sol
  Following the principle that clearer code is better code, consider using the named import syntax (import {A, B, C} from "X") to explicitly declare which contracts are being imported.

Changes

Made explicit imports for these three.