CMake: add settings DD4HEP_BUILD_PACKAGES to chose which packages to build, DD4HEP_BUILD_EXAMPLES to build examples

it will be the responsible of the user to chose a working combination and order of packages
Use DD4hep::<Package> so that cmake will fail if required package was not enabled

Examples: fix order of linking against libraries to avoid picking up old dd4hep headers from lcg installation

listcomponents: rename variable lib to alib to fix shadow warning  (because of lambda capture in c++14?)