Composite projects: support strict dependencies

Google (and Bazel rules_typescript) use a special-purpose TypeScript compiler. One of the reasons for this is we want to enforce strict dependencies.

Strict dependencies means you may only import or reference symbols declared in a project you explicitly declare to be an input. This is an essential property in a large monorepo as a defense against accidental coupling between projects.

For example we might have three directories, A B and C:

alexeagle@alexeagle:~/Projects/repro_strict_deps$ cat A/a.ts
export const a = 'hello';
alexeagle@alexeagle:~/Projects/repro_strict_deps$ cat A/tsconfig.json 
{
    "compilerOptions": {
        "composite": true
    }
}

alexeagle@alexeagle:~/Projects/repro_strict_deps$ cat B/b.ts
import {a} from '../A/a';

export function sayHello(f: string) {
    console.log(a + f);
}
alexeagle@alexeagle:~/Projects/repro_strict_deps$ cat B/tsconfig.json
{
    "compilerOptions": {
        "composite": true,
    },
    "references": [
        {"path": "../A"}
    ]
}

alexeagle@alexeagle:~/Projects/repro_strict_deps$ cat C/c.ts
import {a} from '../A/a'; // SHOULD FAIL HERE
import {sayHello} from '../B/b';

sayHello('world');
console.error(a);
alexeagle@alexeagle:~/Projects/repro_strict_deps$ cat C/tsconfig.json
{
    "references": [
        // NO REF TO A
        {"path": "../B"}
    ]
}

Let's say we work on library B. We decide we no longer need to depend on A, so we remove the dependency, but this breaks users such as C. This is a serious impediment to good code practices because our dependency on A leaked to users who reference its symbols without declaring their own dependency. In a big monorepo, the breakage of removing a dependency can prevent ever cleaning them up

We would like to propose a strictness setting when used with Project References, such that imports (or ambient references) from transitive dependencies should be disallowed.

/cc @evmar @josephperrott

Beginne damit, das Beispiel mit A/a.ts, B/b.ts und C/c.ts sowie dessen tsconfig.json-Projektreferenzen zu reproduzieren. Lies das aktuelle Verhalten von composite-project und project-reference und lege anschließend fest, wie eine Strictness-Einstellung den direkten Import von A durch C ablehnen soll, während die deklarierte Abhängigkeit von B weiterhin zulässig ist. Fertig ist die Aufgabe, wenn der transitive Import oder die ambient reference diagnostiziert wird, sobald keine direkte Referenz deklariert ist.