move experiments/systems

[scheibelp]

Doesn't yet update docs

• Move all static system/experiment definitions into legacy dir
• Move var/exp_repo/experiments to experiments
  • Likewise for var/sys_repo/systems
• Update code references

The actual code changes are small

[scheibelp]

Doc references have been updated:

• I definitely removed all instances of exp_repo/sys_repo
• I think I found all cases where configs referred to the location of system definitions prior to this PR
  • And likewise updated references to the static experiment definitions
  • this required more care since there were legitimate references to "configs"
• There were several references to

    experiments_root/
        ramble/
        spack/
        <Benchmark/ProgrammingModel>/
            <System>/
                workspace/
                    configs/
                        (everything from source/configs/<System>)
                        (everything from source/experiments/<Benchmark/ProgrammingModel>)

where I replaced source/configs/<System> with system_def_dir (since that can come from a legacy static definition or a dynamic one)