Replace abstract NameResolver contract with import in ReverseRegistrar.sol

[AboldUSER]

This pull replaces the abstract contract named NameResolver in the file ReverseRegistrat.sol with an import of the NameResolver from it's respective .sol file. I've come across hardhat compiler issues when using the ens-contracts npm package in my own projects due to two NameResolver contracts existing.

[jefflau]

Thanks for this! Sorry we've been busy and I haven't been able to check external PRs for a while. Happy to merge this if we can resolve the conflicts!

[AboldUSER]

Hey @jefflau I rebased my previous changes on top of the current ens-contracts master branch and looks like the conflicts resolved. Let me know if anything else is needed for this pr!