perf: check existsSync before calling unlinkSync

[raycohen]

unlinkSync is very expensive when it throws, which it will do if existsSync returns false for the path. In my large ember app this gets hit a lot. Note that almost all of the time spent in unlinkSync comes from error handling, which is only needed when unlinkSync would throw. Adding this guard shaves about 4 seconds off of the 21 seconds spent in _copy. I suspect there are more performance improvements to be made here.

These tests were done with node v16.16. The fs error handling behavior/perf might vary across node versions.

Before:
unlinkSync takes 4.9s of 21s in _copy

After:
unlinkSync takes 0s of 17s in _copy