Organize Build*.cmake in different folder inside the superbuild cmake folder

[liesrock]

It looks relatively easy to organize the Build*.cmake files in different folders inside the superbuild cmake project (thanks @traversaro): given the folder organization just update the CMAKE_MODULE_PATH e.g.

list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")

FYI (@EnricoMingo @alaurenzi @pepperig )