@PatrickAlphaC

Yup got the same error today.

hardcoded way to pass this issue for now is create interface directory and there make 3 files: AggregatorInterface.sol, AggregatorV3Interface.sol and AggregatorV2V3Interface.sol. copy content of them from https://github.com/smartcontractkit/chainlink/tree/develop/contracts/src/v0.8/interfaces
then in your MockV3Aggregator.sol import from your local environment. That work for me.

I had this same issue. I just kept toying with it and fortunately it worked.

Well, Patrick is using Chainlink v0.4.0 package and it does not have MockV3Aggregator in v0.8 contracts and that's why you are getting the error that the MockV3Aggregator is not found. If you want to use MockV3Aggregator in v0.8 then you need to upgrade your Chainlink packages to v0.4.2 by yarn add @chainlink/contracts@0.4.2 or simply changing the version in your package.json and run yarn install. v0.4.2 contains MockV3Aggregator in v0.8 contracts.
I think I will create PR for this tomorrow.

The MockV3Aggregator is a mock contract often used for testing in decentralized finance (DeFi) projects, specifically when working with Chainlink price feeds. It allows developers to simulate price feed data without interacting with the actual Chainlink oracles. In Solidity version 0.8, you can use this mock to test your contracts without relying on live price feeds.

Here is a basic example of how to set up the MockV3Aggregator in Solidity 0.8:
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@chainlink/contracts/src/v0.8/tests/MockV3Aggregator.sol";

contract MyContract {
MockV3Aggregator public priceFeed;

constructor(uint8 _decimals, int256 _initialAnswer) {
    // Initialize the mock aggregator with decimals and an initial price
    priceFeed = new MockV3Aggregator(_decimals, _initialAnswer);
}

function getLatestPrice() public view returns (int256) {
    // Fetch the latest price from the mock price feed
    return priceFeed.latestAnswer();
}

}
Key Elements:
MockV3Aggregator: The mock contract imported from Chainlink's tests folder, used for simulating price feed data.
Decimals and Initial Answer: You can set the decimals and initial price data for your mock feed in the constructor.
This is useful for unit testing your smart contracts that rely on Chainlink price feeds. You can easily adjust the price and simulate different scenarios without using live oracles...