The -importmap option takes an argument of the form old=new
and specifies that import "old" should be interpreted as if it said
import "new". The option may be repeated to specify multiple mappings.

This option is here to support the go command's new -vendor flag.

Change-Id: I31b4ed4249b549982a720bf61bb230462b33c59b
Reviewed-on: https://go-review.googlesource.com/10922Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>