module: open stat/readPackage to mutations

[arcanis]

While the CJS loader uses the fs module to perform most filesystem I/O, it uses two hidden internal primitives for speed: internalModuleStat, and packageJsonReader. Because internalModuleStat is destructured, it cannot be replaced. As a result, tools that experiment with virtual filesystem layers have to reimplement the whole CJS resolution stack. This is problematic because we want to experiment writing VFS with https://github.com/nodejs/single-executable.

This diff aims to provide a way for advanced tooling authors to avoid having to hijack the full resolution (which is much more invasive and dangerous than what this diff allows), while still respecting the wish for speed from Node (ie we're not suggesting to stop using the special internal primitives, but just to expose them the same way we already expose Module._findPaths or Module._resolveFilename).