autoproj should ask to delete a source directory if its importer changed

Currently, if e.g. the importer changes from archive to git, the git importer will complain that the source directory is not a git repository. Autoproj/autobuild should have a mechanism where (1) this situation is detected and (2) it asks the user it is OK to delete the source directory.

This is a prerequisite for rock-core/package_set#14
