Download plugins to ~/.local/share/packer instead of ~/.config/packer

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#### Description

Currently `packer init` downloads plugins to ~/.config/packer. However, this directory is intended for configuration files, not platform-specific executables as per [XDG Base Directory Specification](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html). Please consider using ~/.local/share/packer for this.