Offline split vs trig qualifier

[gaponenko]

We need to provide a "non-Geant" build of Offline, and this is currently done using the "trig" qualifier, PR#1179. A cleaner solution is to factor the Offline code into proper pieces. I think the minimal number of repositories is three instead of one:

• The shared code needed by both of the following builds,
• Simulation (Mu2eG4, event generators, digitization), nothing from reco
• Reconstruction

Then one would use a build of Reconstruction instead of the "trig" build. And one would also be able to develop simulations without the burden of compiling all the irrelevant reco code.