Use a graph to keep and search dependencies.

This make it possible to detect that a dependency has been loaded importing a package.

People can add this to their code:

```go
import(
    "github.com/d2fn/gopack/model"
    "github.com/d2fn/gopack/graph"
)
```

And only need to declare one dependency in their `gopack.config`:

```go
[deps.gopack]
  import = "github.com/d2fn/gopack"
```