This allows compiling development packages partially (e.g. using a
non-standard CMake target) to save time when developing locally.

It also allows packages to depend on "variants" of other packages, e.g.
O2Physics -> O2#Analysis and O2sim -> O2#Simulation. aliBuild merges variants
if multiple are required, e.g. aliBuild build O2sim O2Physics in the above
case would build O2 with ALIBUILD_BUILD_VARIANT=Analysis,Simulation. "Full"
builds (no variants) always take precedence, and are the only possible
behaviour for non-development packages (e.g. to allow reusing tarballs as
before, and to avoid heisenbugs in CI).