Importing a local solidity file from its package name can fail in unexpected ways when using a monorepo

[alcuadrado]

This is a bug report of an edge case that can only happen if you have a hardhat project inside a monorepo, and the hardhat project is also an npm project p.

If a solidity file within p imports another one using import "p/contracts/...", the compilation will fail. The reason for this is that the file ends up with two source names p/contract/... and contracts/....

This only happens within a monorepo, because otherwise require.resolve("p/...") would fail. In a monorepo it resolves because the top node_modules will have a symlink to p.

We should forbid importing a file using its own npm project name. We should tell the user which import to use instead.

This bug can be reproduced like this:

1. clone https://github.com/alcuadrado/balancer-core-v2 -- it has a faulty import in pkg/pool-weighted/contracts/smart/LiquidityBootstrappingPool.sol
2. cd into the project
3. git checkout compilation-bug-reproduction
4. yarn
5. cd pkg/pool-weighted
6. yarn hardhat compile --show-stack-traces --verbose

[alcuadrado]

The best place to forbid this kind of imports is probably here: https://github.com/nomiclabs/hardhat/blob/master/packages/hardhat-core/src/internal/solidity/resolver.ts#L94