Ask confirmation if attempting to build release --upgrade without changed mix version

### Description of issue

Whenever you've created a release, and are building a new one that you want to use for a hot-upgrade, if you build this one without changing your mix version, then stuff goes wrong once you attempt to upgrade.
To complicate things further, one you've encountered this, bumping the mix version afterwards and building a new release will create an .appup that is worthless.

The only 'simple' way to fix this problem is to perform a hard (non-hot) upgrade.

### Proposed fix

If, when the user would attempt to execute this, and Distillery realizes that there already is a release for this app version, it would ask for a confirmation, then this gives us an explicit way to solve the issue before it becomes a problem.

Start with the `release --upgrade` build path and how it detects an existing release for the same app version. Trace the handling of the Mix version and generated `.appup` files; done means the user receives a confirmation before proceeding when that version already exists, preventing the described invalid upgrade path.

Written by the indexing model from the issue text.