Don't recreate links that already exist

[Corecii]

Recreating links that already exist can cause failure if any referenced tool is in use. This is common with Rojo and serving or watching for sourcemap changes.

Here's an example:

Re-writing links is not necessary. Similarly, re-writing already-installed tools is not necessary, but I don't actually know if rokit does that!

This has been an annoying issue with aftman, and it would be great to see it fixed in rokit!

[filiptibell]

I think we can fix this. Rokit currently always re-creates links since each "link" is really just a copy of Rokit itself, and if Rokit updates, we obviously want the links (fake Rokits) to update too. What we could do though, is:

1. Attach some metadata to the end of each link binary, that contains the Rokit version
2. Seek the end of each link during link re-creation for this metadata
3. If the version matches the current binary that is trying to re-create the links, we skip