* pyenv-binary: add experimental plugin skeleton
Adds a new plugin, decoupled from `pyenv install`, for packaging and installing
relocatable Python binaries. This commit is just the command dispatcher; the
individual subcommands follow.
`pyenv binary <command>` dispatches to the matching script under the plugin's
libexec, so subcommands stay out of the top-level `pyenv commands` list.
Groundwork for the binary distribution support discussed in #2334.
* pyenv-binary: add the `save` subcommand
`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).
* pyenv-binary: reject invalid version names and drop readlink -f in save
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: test the `save` subcommand
Cover argument validation, the version-name guard, and packaging an
installed version into an archive with matching metadata.
* pyenv-binary: test dependency parsing and tighten the version guard
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.
