"lb-run --nightly" should directly use the projects from the nightlies slot and not try to find them

Today lb-run --nightly set the search path up so that the lookup of projects would find the projects in the nightly build deployment.

Although this works relatively well, it requires that the nightly builds are deployed using directories including the projects versions (currently worked around with symlinks, but not possible with the new nightly builds system under commissioning). A more correct approach si to skip the lookup for the projects in the slot and directly use the content of the nightly build deployment.

To be more precise, in the example of the slot test-slot/42 containing Gaudi/master (internal version 36.3) and LHCb/HEAD (internal version 53.5), we deploy in the directory /cvmfs/lhcbdev.cern.ch/nightlies/test-slot/42 the projects in the directories Gaudi and LHCb. Invoking lb-run --nightly test-slot/42 LHCb does not work unless you also specify the platform, so we usually go for lb-run --nightly test-slot/42 LHCb/HEAD, which requires that we pair the directory LHCb with the symlink LHCb_HEAD. The metadata of the installed LHCb claim that it needs Gaudi/36.3 as declared by Gaudi's internal version so we need to pair the directory Gaudi with the symlink Gaudi_v36r3 to please the lookup algorithm. The deployment of the new nightly builds system cannot create those symlinks for technical reasons beyond the scope of this issue, but they are not really needed as we know upfront exactly which installation directory we need for each project in the slot, so there is no need for the lookup.

This is partially related to #36 as a nightly build installation is also a "flat project structure" (or a deployment without version directories).