fix(plugin-npm): mark fetchFromNetwork private to fix TypeScript issue

[vinzscam]

What's the problem this PR addresses?

As part of developing a Yarn plugin for our project, we introduced @yarnpkg/plugin-npm as a dependency. However when running typescript, the package raises the following error:

node_modules/@yarnpkg/plugin-npm/lib/NpmHttpFetcher.d.ts:12:76 - error TS2307: Cannot find module '/home/runner/work/berry/berry/.yarn/__virtual__/@yarnpkg-libzip-virtual-4957b34c08/1/packages/yarnpkg-libzip' or its corresponding type declarations.

12     fetchFromNetwork(locator: Locator, opts: FetchOptions): Promise<import("/home/runner/work/berry/berry/.yarn/__virtual__/@yarnpkg-libzip-virtual-4957b34c08/1/packages/yarnpkg-libzip").ZipFS>;

I've looked at the generated lib/NpmHttpFetcher.d.ts file https://www.npmjs.com/package/@yarnpkg/plugin-npm?activeTab=code, I found that the issue was in the fetchFromNetwork signature which includes a path generated during CI (/home/runner/work/berry/berry/...), which cannot be resolved locally.

export declare class NpmHttpFetcher implements Fetcher {
    supports(locator: Locator, opts: MinimalFetchOptions): boolean;
    getLocalPath(locator: Locator, opts: FetchOptions): null;
    fetch(locator: Locator, opts: FetchOptions): Promise<{
        packageFs: import("@yarnpkg/fslib").FakeFS<import("@yarnpkg/fslib").PortablePath>;
        releaseFs: () => void;
        prefixPath: import("@yarnpkg/fslib").PortablePath;
        checksum: string | null;
    }>;
    fetchFromNetwork(locator: Locator, opts: FetchOptions): Promise<import("/home/runner/work/berry/berry/.yarn/__virtual__/@yarnpkg-libzip-virtual-4957b34c08/1/packages/yarnpkg-libzip").ZipFS>;

How did you fix it?

As the fetchFromNetwork method seems to be intended for local use only, I've marked it as private so that TypeScript skips inferring the return type, marking the change as minor. But please let me know if you would prefer to keep the method public and explicitly define the return type.

I've spotted a similar error in:

• berry/packages/plugin-github/sources/GithubFetcher.ts

  Line 39 in 1908ee7

  async fetchFromNetwork(locator: Locator, opts: FetchOptions) {

• berry/packages/plugin-http/sources/TarballHttpFetcher.ts

  Line 34 in 1908ee7

  async fetchFromNetwork(locator: Locator, opts: FetchOptions) {

happy to fix those as well.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.