Add tests that feed realistic ldd and otool listings through save and
check only the libraries resolving outside the prefix end up in the
metadata, since that filtering is the fiddliest part of the command.
Narrow the version guard to reject `.' and `..' rather than any name
containing a dot-dot, now that a slash is already refused, and note why
the find still matches *.so.* even though CPython does not produce them.
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).
