Clone Spack (and Ramble) less

[scheibelp]

Clone spack and ramble once per benchpark workspace (vs. once per benchmark workspace).

For example

benchpark setup saxpy/openmp x86 test-workspace
benchpark setup raja-perf/openmp x86 test-workspace

would have normally cloned Spack and Ramble twice (two copies of each).

This moves Spack and Ramble into the benchpark workspace dir (i.e. workspace_root in benchpark setup --help). With the changes here, the test-workspace dir would look like:

$ ls test-workspace/
raja-perf  ramble  saxpy  spack

which might be confusing: perhaps ramble and spack could be put into a tools or libs subdirectory.