Manage project subdirectories without pulling the project as a dependency

Don't you hate when your project doesn't compile because you're using a global repo subpath to import a dependency? Me to.

So, instead of adding a new dependency, you can declare your repository name, and gopack will link your source in the right place.

You can still write idiomatic go to import a directory from your own project using something like:

```go
import "github.com/calavera/foo/bar"
```

Without pulling down the remote repo and using your local source, including this in your `gopack.config`:

```toml
repo = "github.com/calavera/foo"
```

/cc @d2fn