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

まず、tsconfig.json の project references を含む A/a.ts、B/b.ts、C/c.ts の例を再現します。現在の composite-project と project-reference の動作を読み、そのうえで、strictness 設定によって C から A への直接インポートを拒否しつつ、B への宣言済み依存関係は許可する方法を定義します。直接参照が宣言されていない場合に、推移的インポートまたは ambient reference が診断されれば完了です。