Allow using local packages during build
Currently we expect user to use aliBuild (and environment variables containing _ROOT) in order to configure and build MpdRoot. This is not correct approach, especially if user wants to use local packages. We will change the logic in the sense, packages will be searched in the order:
- Packages provided by user via -DPACKAGE_ROOT
- Search for local packages
- Search for environment variables If none of the previous steps will lead to locating package, we will give an error message.