Turn find_package() into no-op if looking for builtins

From now on, if a package Foo is built by ROOT itself, its name should
be added to the 'ROOT_BUILTINS' list, which turns find_package(Foo) into
a no-op. This prevents find_package(Foo) from wrongly finding a version
of the package already installed in the system, which would lead to
inconsistencies in the build. Packages moving to this new system must
make sure to setup all variables that would normally be set up by the
call to find_package(), including the creation of imported targets with
the appropriate properties (i.e., include directories, etc).