x/tools/cmd/goimports: package clause insertion mangles comments

Input:

``` go
// a
// b
// c

func main() {
    fmt.Println()
}
```

gofmt fails appropriately:

```
$ gofmt x.go 
x.go:5:1: expected 'package', found 'func'
```

goimports inserts a package clause and mangles the comments:

```
$ goimports x.go 
package main // a
import "fmt"

// b
// c

func main() {
    fmt.Println()
}
```
