Consider replacing internal compose package + files with duct

Internal Docker Compose package: https://github.com/elastic/elastic-package/tree/master/internal/compose
Internal Docker Compose files: https://github.com/elastic/elastic-package/tree/master/internal/install
Duct: https://github.com/erikh/duct

Note that Duct is fairly new so we might want it to mature a bit before we adopt it. Also, not that it is not a drop-in replacement for a Docker Compose SDK; it's a complete programmatic replacement for Docker Compose (so no `.yml` files, but `struct`s instead, etc.).