Add support for imports with explicit package name

[jpap]

Thanks for a great tool!

In using it for golang.org/x/tools, I found gomove missed imports that use an explicit package name:

import _ "a/package/name1"       // side-effects-only import
import myname "a/package/name2"  // imported as myname

// Alternatively...
import (
 "an/unrelated/package"

  _ "a/package/name1"
  myname "a/package/name2"
)