Fix Hardhat compile error when public variables are used to implement interface functions

[ericglau]

When modifying solc input to extract namespaced storage layouts, interface functions are kept (because constants might refer to their function selectors), but we effectively remove state variables (by converting them to dummy enums).

If an interface has a function, which is implemented using a state variable to provide a public getter, then this causes a compile error because the modified solc input (with state variable converted to dummy enum) would no longer have the public getter.

We can resolve this by converting all contracts in the modified solc input to abstract.

Fixes #1053

[Amxx]

I was worried something like

contract HasExternalViewFunction {
    uint256 public foo;
    bytes4 constant USING_INTERFACE_FUNCTION_SELECTOR = this.foo.selector;
}

would be an issue, but apparently this is not valid solidity.

this is valid, but since its not constant I'm guessing it get removed, right?

    bytes4 immutable USING_INTERFACE_FUNCTION_SELECTOR = this.foo.selector;

[ericglau]

Added a testcase for the above.

Immutables are effectively removed. Since we are converting all contracts to abstract now, removing immutables also avoids the following from being an issue:

contract DeploysContractToImmutable {
    MyContract public immutable myContract = new MyContract();
}