Add on-demand vendoring for packages / proper dependency management

[fschutt]

Right now the dependency management in the wasmer.toml is in an infancy state, we need to figure out how / when to vendor packages to make them self-contained on-demand.

We should also test that things like std::process::Command::new("python") works from inside a natively-compiled executable, if said executable is vendored. This means that process / environment management has to be aware of all the commands that are currently available inside of the container (could be difficult).

[syrusakbary]

This issue can be split into two sides:

• How packaging can describe and use dependencies. We need an architecture document that @Michael-F-Bryan will handle
• How dependencies will be downloaded when using a package. It seems this is partially achieved in WASIX in the wasix branch