A version is a single directory name under versions/, so refuse names with a
slash or a dot-dot component before building the prefix path.
Also emit the python paths directly instead of `readlink -f`, which BSD
readlink on macOS does not support; ldd/otool follow the symlinks anyway, so
the resolved paths were never needed.
`pyenv binary save <version> [<output-dir>]` packs an installed version into a
relocatable .tar.gz with relative paths and writes a metadata file recording the
build platform, distro, libc version and the external system libraries the
build links against (via `ldd` on Linux, `otool -L` on macOS).
