Enhancement: [no-unsafe-declaration-merging] Switch to scope manager

[JoshuaKGoldberg]

Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Link to the rule's documentation

https://typescript-eslint.io/rules/no-unsafe-declaration-merging/

Description

Per #5840 (comment):

We should actually be able to do this without type info and just with the scope analysis APIs.

Not using type information where possible is preferable for rules. See https://typescript-eslint.io/docs/linting/typed-linting/.

Fail

(same as before)

Pass

(same as before)

Additional Info

This rule is only in the strict config, not one of the recommended ones (yet). So it wouldn't be a breaking change to use the scope manager.

[bradzacher]

It's worth noting that by switching from type information we won't be able to detect these two cases:

namespace Foo {
  export interface Bar {}
}
namespace Foo {
  export class Bar {}
}

import Foo from './Foo';

namespace Foo {
  export interface Bar {}
}

I don't think that's a huge problem given that it would vastly improve the performance and usability of the rule. Those are pretty rare edge-cases as well.