highlight the use of DPM_SDK_VERSION

perhaps in the output of `dpm versions` list (which is meant for humans)

```
dpm version
   3.4.0-stuff-123
 * 3.4.0-stuff-456 (DPM_SDK_VERSION)
   3.4.0-stuff-789
```

or emit a warn log to stderr

(Or maybe we can get rid of this feature altogether. https://github.com/DACH-NY/dpm/issues/479)