Only install @parity/hardhat-polkadot if hardhat is also a dependency

[BigTava]

🤔 Expected Behavior

People think @parity/hardhat-polkadot is a plug and play library with existing hardhat setups.

😯 Current Behavior

People think @parity/hardhat-polkadot is a fork of hardhat

[ryanleecode]

We already have hardhat as a peer dep so i dont see what else can be done here