Fixes to support re-exported imports and additional tests

[cd1m0]

This pr adds support for re-exported imports and unit alias imports. E.g given the following 3 files:

A.sol
contract Foo {..}

B.sol
import "A.sol" as A;
import {Foo as Boo} from "A.sol";

C.sol
import {A as A1} from "B.sol";
import {Boo as Moo} from "B.sol";
import "B.sol";

contract Test{
     A1.Foo x;
     Moo y;
     Boo z;
}

When re-writing in flat mode, all the different aliases for Foo - A1.Foo, Boo, Moo should just be rewritten to Foo:

pragma solidity 0.7.5;

contract Foo {...}
...
contract Test {
    Foo internal x;
    Foo internal y;
    Foo internal z;
}

Notes:
- this PR also fixes a bug in the grammar for imports
- it introduces the replaceNode function in scribble.ts. That function should eventually be moved to solc-typed-ast.

[blitz-1306]

Looks good.