add BSC's hsmcompute site-specific config

• add BSC's hsmcompute config from !445 (merged)
• re-structure the config files: create separate packages.yaml, compilers.yaml, repos.yaml and only keep "extra" specs needed for BSC in spack.yaml (that will be merged with the top-level spack.yaml)

mentioned in commit bdd98655

merged

mentioned in merge request !445 (merged)