x/tools/cmd/goimports: comment movement during import add

[josharian]

Migrated from #6982 comment 6:

http://play.golang.org/p/vXaSQ_EUUg

Before:

package main

import (
        "io" // I Am the Very Model of a Modern Major-General
)

var _ = io.Copy

func main() {
        log.Println("Hi")
}

After:

package main

import (
    "io"
    "log" // I Am the Very Model of a Modern Major-General
)

var _ = io.Copy

func main() {
    log.Println("Hi")
}

Note that the "io" import comment is now next to "log".

[gopherbot]

CL https://golang.org/cl/19822 mentions this issue.