Make node index cache aware of the cached URL

[charlespierce]

Closes #242

Info

• The current cache of the Node index is global and doesn't know about the fact that hooks can affect the URL we check for the Node index.
• This means that if you create the cache using the public repo, but then switch to a project that has hooks pointing to a private repo, the cache used will still be that of the public repo.
• Problems can arise when a private repo has a subset of the publicly available versions of Node, because the cache could result in us writing an invalid Node version into package.json, even though the project is correctly configured with Volta hooks.

Changes

• Updated the cache format to include the URL that was fetched as the first line, followed by the contents.
• Updated the function that reads the cache to check for the URL at the start and only return the contents if the cached URL matches the requested URL.
• Added debug output of whether or not the cache was used.

Tested

• Local testing to confirm that switching between a project with hooks and the public repo results in the cache resetting, while staying in a single context means the cache is used.

[chriskrycho]

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