Consider replacing `fs` calls with a lib that deals with OS specific edge cases.

[rally25rs]

There is a fair amount of code in fs.js (or fs-normalized.js once #5470 is accepted) that deals with oddities in case sensitive, timestamp preservation, file access differences across OSs and Node versions.

We should explore simplifying this by using a 3rd party library instead.
Perhaps https://github.com/jprichardson/node-fs-extra
which seems to do checks for the existance of fs.copyFile and timestamp normalization.

Need to evaluate performance on several OS's (OSX, Win, Linux) and node versions (pre and post 8.5.0)

[arcanis]

Yep I'd support that. Once advantage of fs-extra is that it automatically wraps everything to return promises, which would allow us to remove at least this part of fs.

[paul-soporan]

Closing this as we now use @yarnpkg/fslib in the v2 that handles everything in a type-safe way.