unifyfs: remove flatcc dependency and add spath (#17913)
unifyfs: remove flatcc dependency and add spath (#17913)

FlatCC has been removed from UnifyFS as a dependency on the develop
branch and for future releases.

spath is now an optional dependency for UnifyFS to normalize relative
paths provided by the user.
iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack