Be way more cautious about overwriting existing files.

[beporter]

The installer is currently extremely unceremonious about overwriting any existing files in a consuming project. If you have your own Vagrantfile in your project root, and you composer require loadsys/puphpet-release, that file will get overwritten without notice.

This is, Bad™, in no uncertain terms. We should attempt to detect existing files on our FIRST installation only, and abort (or prompt) if they are already present.

As an important distinction; on subsequent installs (during composer updates) it should be safer to assume that /Vagrantfile and /puphpet/ are now under "our" control, and can be updated immediately.

This ties into how we want to handle PuPHPet's files/ and exec-*/startup-* folders though, since currently there is no mechanism for integrating those from the consuming project into the "installed" /puphpet folder.