ImportResolver's PROVIDERS fiield is private

[rainman-wf]

PROVIDERS contains only ["@openzeppelin/contracts", "@uniswap/lib"] deps, but I nead also @openzeppelin/contracts-upgradable and hardhat. And now a way to set required parameters то this set.

I think it is necessary to take out a setter for this parameter in Web3jExtension class. Also, you need to solve the problem of specifying a custom path to the sol. source files

Your plugin is good, guys! But not flexible enough.