func NewPackager(config Config) *Packager {
	config.populateDefaults()
	return &Packager{config}
}
NewPackager creates and initializes a new Packager with the provided
Config. It panics when config.ExchangeFactory is nil.
package webpackager
import (
	"net/http"
	"net/url"
	"time"
	"github.com/google/webpackager/resource"
	"golang.org/x/xerrors"
)
type Packager struct {
	Config
}