[prefer-readonly-parameter-types] IndexSignature internals don't violate the rule

[RebeccaStevens]

I wasn't sure if this was intentional behavior or not so I thought I may as well open an issue on it.

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have read the FAQ and my problem is not listed.

Repro

{
  "rules": {
    "@typescript-eslint/prefer-readonly-parameter-type": "error"
  }
}

type Foo = {
  readonly [key: string]: {
    groups: string[];
  };
};

function foo(arg: Foo) {}

Expected Result

The PropertySignature in the IndexSignature is not readonly and thus the rule should be violated.

Actual Result

No rule violation.

Versions

package	version
@typescript-eslint/eslint-plugin	4.29.0
@typescript-eslint/parser	4.29.0
TypeScript	4.3.5
ESLint	7.27.0
node	16.4.0

[kmin-jeong]

can I try it?