Add validation checking that modules of used external identifiers are imported

[bbannier]

We currently allow the implementation of hooks defined in another module without requiring the module to be imported, e.g., given a module a

# a.hlt
module a {

import hilti;

declare public hook void h();

h();

function hook void h() { hilti::print("a::h (a)"); }

}

we can declare a module b

# b.hlt
module b {

# `a` not explicitly imported.
import hilti;

public type a = struct {
    hook void h();
};

# Implementation for undeclared hook from module `a`.
function hook void a::h() { hilti::print("a::h (b)"); }

method hook void a::h() { hilti::print("b::a::h"); }

global b::a x;
x.h();

}

If we compile b.hlt with a.hlt the hook declaration from a becomes visible and produces the expected output,

$ hiltic -j b.hlt a.hlt
b::a::h
a::h (b)
a::h (a)

If we compile b.hlt standalone the undeclared hook is not diagnosed,

$ hiltic b.hlt -j
b::a::h

Above example contains another twist in that if b.hlt actually contained an import a; a redefinition error would be triggered

$ hiltic b.hlt -j
[error] b.hlt:6:1-8:3: redefinition of 'a' defined in b.hlt:4:1
[error] hiltic: aborting after errors

In order to make e.g., codegen more deterministic we should probably make an explicit import mandatory when accessing external names.