[llvm] [cmake] Move LLVM_INSTALL_PACKAGE_DIR top-level to fix llvm-config

Move the `LLVM_INSTALL_PACKAGE_DIR` declaration from llvm/cmake/modules
directory to the top-level llvm/CMakeLists.txt, in order to fix
the regression in `llvm-config --cmakedir` output for installed LLVM.
Since the tools directory is processed by CMake prior to
llvm/cmake/modules, the llvm-config executable ended up using
the variable prior to it being defined and incorrectly used an empty
path, resulting in e.g.:

    $ llvm-config --cmakedir
    /usr/lib/llvm/16/

With this patch, the path is defined (and therefore the default value
is being set) prior to adding the tools subdirectory and llvm-config
starts working correctly:

    $ llvm-config --cmakedir
    /usr/lib/llvm/16/lib64/cmake/llvm

This fixes a regression introduced by D130539.  Thanks to Petr Polezhaev
for reporting the problem @ https://bugs.gentoo.org/865165

Differential Revision: https://reviews.llvm.org/D131878