Declarations of "unknown" modules aren't included despite enabled inlineDeclareExternals flag

[timocov]

Bug report

Input code

// config.ts
const config: TestCaseConfig = {
	output: {
		inlineDeclareExternals: true,
	},
};

index.d.ts:

/// <reference path="./modules.d.ts" preserve="true" />

export {};

modules.d.ts:

declare module '*.css' {}

declare module 'module-that-does-not-exist-actually' {
}

Expected output

declare module "*.css" { }
declare module "module-that-does-not-exist-actually" {
}

export {};

Actual output

export {};

Additional context
Add any other context about the problem here (CLI options, etc)