Duplicate declare module '@sapper/app' in src/node_modules/@sapper/index.d.ts

[acim]

Describe the bug

When I run 'npm run dev' or "npm run build', 'src/node_modules/@sapper/index.d.ts' gets generated, but it contains double declaration of all 4 modules. Here is how the file looks like:

declare module '@sapper/app'
declare module '@sapper/server'
declare module '@sapper/service-worker'
declare module '@sapper/common'

declare module '@sapper/app' {
	export interface Redirect {
		statusCode: number
		location: string
	}

	export function goto(href: string, opts: { noscroll?: boolean, replaceState?: boolean }): Promise<void>;
	export function prefetch(href: string): Promise<{ redirect?: Redirect; data?: unknown }>;
	export function prefetchRoutes(pathnames: string[]): Promise<void>;
	export function start(opts: { target: Node }): Promise<void>;
	export const stores: () => unknown;
}

declare module '@sapper/server' {
	import { Handler, Req, Res } from '@sapper/internal/manifest-server';

	export type Ignore = string | RegExp | ((uri: string) => boolean) | Ignore[];

	export interface MiddlewareOptions {
		session?: (req: Req, res: Res) => unknown
		ignore?: Ignore
	}

	export function middleware(opts: MiddlewareOptions): Handler;
}

declare module '@sapper/service-worker' {
	export const timestamp: number;
	export const files: string[];
	export const assets: string[];
	export const shell: string[];
	export const routes: Array<{ pattern: RegExp }>;
}

declare module '@sapper/common' {
	export interface PreloadContext {
		fetch: (url: string, options?: any) => Promise<any>;
		error: (statusCode: number, message: Error | string) => void;
		redirect: (statusCode: number, location: string) => void;
	}

	export interface Page {
		host: string;
		path: string;
		params: Record<string, string>;
		query: Record<string, string | string[]>;
		error?: Error;
	}

	export interface Preload {
		(this: PreloadContext, page: Page, session: any): object | Promise<object>;
	}
}

This further causes that these 4 modules are not properly recognized. For example, if I do:

<script context="module">
	import type { Preload } from "@sapper/common";

	export const preload: Preload = async function(this, page, session) {
		const { user } = session;

		if (!user) {
			return this.redirect(302, 'login'); // TypeScript will know the type of `this` now
		}

		return { user };
	}
</script>

I get the following error:

Cannot use namespace 'Preload' as a type.ts(2709)

To Reproduce

Just run 'npm run dev' or "npm run build' and look at the 'src/node_modules/@sapper/index.d.ts' file.

Expected behavior
To generate 'src/node_modules/@sapper/index.d.ts' without this block:

declare module '@sapper/app'
declare module '@sapper/server'
declare module '@sapper/service-worker'
declare module '@sapper/common'

Information about your Sapper Installation:

• The output of npx envinfo --system --npmPackages svelte,sapper,rollup,webpack --binaries --browsers

  System:
    OS: Linux 5.9 Linux Mint 20 (Ulyana)
    CPU: (16) x64 Intel(R) Core(TM) i9-9900K CPU @ 3.60GHz
    Memory: 24.27 GB / 30.90 GB
    Container: Yes
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 14.13.1 - /usr/bin/node
    Yarn: 1.22.4 - /usr/bin/yarn
    npm: 6.14.8 - /usr/bin/npm
  Browsers:
    Firefox: 81.0
  npmPackages:
    rollup: ^2.3.4 => 2.30.0 
    sapper: ^0.28.0 => 0.28.10 
    svelte: ^3.17.3 => 3.29.0

Severity

Blocks usage of TypeScript.

[singingwolfboy]

I think this is already resolved by #1598 -- it's just not yet in a published release.