# ls repos/spack_repo/builtin/packages/ | head -20
abyss
adept-utils
adios
adios2
adlb
admixtools
...

# spack info zlib
Package: zlib

Versions:  1.3.1, 1.3, 1.2.13, ...
Variants:
    build_system [autotools, makefile]  autotools
    ...
Installation Phases:
    autoreconf, configure, build, install