Cache invalidation API

[cspotcode]

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In ts-node, if someone modifies a .ts file, removes it from their require.cache, and then re-requires it, we will have a new sourcemap for that file. We need a way to invalidate that entry in your cache so that you re-retrieve it from us.

This issue is requesting a cache invalidation API. All we need is a function to call, passing the path of a file that you have previously requested from us, telling you to remove that entry from your cache.