Tact files cannot reference FunC contracts in imports

[anton-trunov]

This is almost like issue #4, but for FunC imports this time. Let's say we have these two files:

The main Tact project:

// main
import "@stdlib/deploy";
import "./native.fc";

struct Pair {
    fst: Int;
    snd: Int;
}

@name(get_pair)
native getPair(): Pair;

contract Native with Deployable {

    init() {}

    receive("go") {
        dump(getPair().fst);
        dump(getPair().snd);
    }
}

And a FunC file:

// native.fc
(int, int) get_pair() {
    return (4, 2);
}

When trying to build main.tact, I get the following error message:

Could not resolve import ./native.fc in /main.tact