[pkg] fetch dependencies without building the project

[maiste]

Context

In the current behaviour, unless you have the exact target, it is impossible to download the dependencies without building the project. A nice feature would be a command that fetch the project dependencies in the same way you would with dune build. This is important in the context of Dockerfile, where you can save some computation time by caching this stage.

Solution

We can implement a dune pkg fetch command that triggers the fetch rule for the entire project.