FindMPI: Fix various legacy problems

This MR fixes a number of bugs stemming from legacy variable handling, partially introduced by !893 (merged).

• MPI_COMPILE_OPTIONS and MPI_COMPILE_DEFINITIONS legacy hints were not passed correctly.
• Legacy variables could be parsed after they were set by FindMPI cause erroneous hinting on subsequent runs.
• In the case of the compiler supporting MPI implicitly:
  • not all consumption variables would be correctly reset.
  • MPI_<LANG>_HEADER_DIR would be set unintentionally.
  • MPI_<LANG>_LIB_NAMES appeared in cache despite being ignored.
  • on subsequent passes, MPI_<LANG>_COMPILER could be corrupted.
• the C-to-CXX copy logic would take place even if MPI_SKIP_GUESSING was turned on
• if only some language compilers were given, inconsistent search behavior could be observed

Issue: #17538 (closed)