Aegir Platform Git submodule can allow duplicate platform paths

This happens because Aegir checks in the validation step of the form to see if the publish path is a duplicate, but Aegir Platform Git changes the publish path in the submit step, thus essentially bypassing this check.

I've marked this issue as critical because anyone who has platforms that end up in this state will need to be extremely careful that they don't migrate sites between them or delete the platforms etc.

Given the lifecycle of Aegir in general, I'm going to fix the common way that this can break, specifically, where the full publish path matches another platform, but isn't correctly validated by this module.

But there are other, more subtle ways to break things here I suspect, like picking the same path for the root of the repo, but changing the docroot. Then it'll let you make that platform, but presumably do odd things like try to delete it etc.?

Fixed my specific use-case.