x/tools/cmd/goimports: external package is preferred over package level variable 

In db.go

``` go
package api

type Stat struct {
}

type DB struct {
    Stats *Stat
}

var (
    store = &DB{&Stat{}}
)
```

In stats.go

``` go
package api

// this line is imported by goimports automatically, where it shouldn't 
import "github.com/coreos/etcd/store" 

func takeStats() {
    s := store.Stats

    s = nil
}
```

go version go1.5rc1 darwin/amd64
Tools revision: 9f2124f (HEAD)
