fix(plugin-npm): mark fetchFromNetwork private to fix TypeScript issue (#6764)

## 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:

```bash
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.

```ts
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:
-
https://github.com/yarnpkg/berry/blob/1908ee79fdf80fc8397d8ff928f3bfe25f30e715/packages/plugin-github/sources/GithubFetcher.ts#L39
-
https://github.com/yarnpkg/berry/blob/1908ee79fdf80fc8397d8ff928f3bfe25f30e715/packages/plugin-http/sources/TarballHttpFetcher.ts#L34

happy to fix those as well.

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---------

Signed-off-by: Vincenzo Scamporlino <[email protected]>

Author: vinzscam

.yarn/versions/3d022429.yml

@@ -0,0 +1,8 @@
+releases:
+  "@yarnpkg/plugin-npm": minor
+
+declined:
+  - "@yarnpkg/plugin-compat"
+  - "@yarnpkg/plugin-npm-cli"
+  - "@yarnpkg/cli"
+  - "@yarnpkg/core"

packages/plugin-npm/sources/NpmHttpFetcher.ts

@@ -43,7 +43,7 @@ export class NpmHttpFetcher implements Fetcher {
     };
   }
 
-  async fetchFromNetwork(locator: Locator, opts: FetchOptions) {
+  private async fetchFromNetwork(locator: Locator, opts: FetchOptions) {
     const {params} = structUtils.parseRange(locator.reference);
     if (params === null || typeof params.__archiveUrl !== `string`)
       throw new Error(`Assertion failed: The archiveUrl querystring parameter should have been available`);