This reworks FindMKL.cmake to model the oneMKL link closure required by ABACUS rather than exposing individual MKL libraries to callers.

ABACUS directly uses LP64 Fortran-style BLAS/LAPACK symbols and, in MPI builds, BLACS/ScaLAPACK routines. The finder now provides:

• abacus::mkl for the BLAS/LAPACK and FFTW compatibility interfaces;
• abacus::mkl_scalapack for the MPI closure, including ScaLAPACK, the matching BLACS library, base MKL libraries, and MPI::MPI_CXX.

The configuration handles the relevant oneMKL choices explicitly:

• LP64 only, since ABACUS integer arguments are not ILP64-compatible;
• mkl_gf_lp64 for GCC and mkl_intel_lp64 otherwise, preserving the established ABACUS interface-library selection;
• Sequential, GNU-threaded, or Intel-threaded MKL;
• Static or shared oneMKL libraries, with a local linker group for static archive closures on Linux;
• Open MPI through mkl_blacs_openmpi_lp64, while MPICH and Intel MPI use mkl_blacs_intelmpi_lp64, as required by oneMKL.

Closes #7568.

Note about visible affects

1. For MKL_MPI=AUTO, MPI library-version probing is enabled before find_package(MPI) when the build is not cross-compiled. This allows Open MPI to be detected reliably, but does not work for cross builds. People who process cross builds must select the BLACS interface explicitly through MKL_MPI when using MKL.
2. If you use MKL with OpenMPI, better be careful of the status message -- oneMKL BLACS interface: ${_mkl_blacs_name} and ensure ${_mkl_blacs_name} here is mkl_blacs_openmpi_lp64; normally this should work fine.
3. Tested compiler sets: GNU, IntelLLVM, LLVM. Other compilers might need care because only GNU uses mkl_gf_lp64.
4. This PR targets tested Linux configurations only (including Windows WSL2) as ABACUS is a Linux-based program anyways.
5. Original cache variables such as MKL_INCLUDE are no longer available to pass manually, as this is the responsibility of internal logic.