Output the reason why package/module was installed

[wclr]

There should be a way to know why a package/module was installed (downloaded) showing which packages depend on it.

We could add an argument to spago ls deps , so e.g. if you want to know all the transitive dependencies of halogen you'd do spago ls deps --transitive halogen

In other package managers there is API called why for example yarn why package-name.

[f-f]

We can get this in after #1069

[f-f]

Thinking about the design of this:

We would support both modules and packages under the same command (we can distinguish easily because modules are capitalised, packages can't be:

spago explain Data.Maybe

spago explain aff

The result could look something like this:

$ spago explain maybe
{
  "module": "Data.Maybe",
  "package": "maybe",
  "dependents": [
    {
      "module": "Data.List",
      "package": "lists",
      "dependents": [
        {
          "module": "Main",
          "package": "my-project"
        }
      ]
    }
  ]
}