Fix import paths for cascadia and x/net/html

Google Code has shut down, so these have been long out of date.  (And
possibly made it impossible to build favicon on other systems?)

go/favicon/favicon.go

	"os"
	"sync"
	"code.google.com/p/cascadia"
	"code.google.com/p/go.net/html"
	"github.com/andybalholm/cascadia"
	"github.com/golang/groupcache/lru"
	"golang.org/x/net/html"
)
// from https://commons.wikimedia.org/wiki/File:1x1.png