Factor out almost all fs. and path. calls in the tool

This will be useful when we want to be smart with windows file paths later
Also, all of the file calls are asynchronous with fibers now, which comes with
many benefits.
The case is: you can use `\` in a unix path. But you can't use `/` in a Windows path. So if Meteor happens to work with paths containing backslashes on Windows (i.e. Meteor tries to create such paths), then it will fail.

Let's not do that.