Import alias

Add alias directive to be able to import forks without modifying every `import` directive in your project.

Given the fowolling gopack.config:

```toml
[deps.mux]
import = "github.com/calavera/mux"
alias  = "github.com/gorilla/mux"
branch = "1.0rc2"
```

Gopack will download the library from `github.com/calavera/mux`, but it will leave it under `github.com/gorilla/mux` for you to import it in your code.

/cc @d2fn