add a command to easily download the modules from a list

[mvdan]

I think it should also have two modes:

1. Only download the source for those modules, and nothing else. Useful for very simple analysis, like counting lines of Go code, applying code formatting changes, or grepping for simple patterns.

2. Download the modules and their dependencies into a module cache, likely with go get -d.

These modules could be called "shallow" and "module cache". Both should use the module proxy, for the sake of being consistent, fast, and reproducible.