Aggressive cleanup leaves broken symlinks

[matthiasrichter]

After running with option --aggressive-cleanup the symlinks for individual packages in sw/TARS/<arch>/<package>/ are broken as the actual tar files in the store are missing.

This leads to a problem if the cached build actually needs to be used. In addition, the error message is wrong, or points to another problem

Using cached build for autotools
ERROR: Error while updating reference repos https://github.com/alisw/autotools.

To provoke this situation I did (somewhat artificial but it boils down the problem I was running into after the aggressive cleanup):

alibuild/aliBuild --aggressive-cleanup clean root
rm sw/ubuntu1604_x86-64/autotools
alibuild/aliBuild --defaults o2 build o2

Build works again if the symlink is removed as well.