go/types: don't exclude package unsafe from a Package's Imports list

go/types: don't exclude package unsafe from a Package's Imports list

There's no good reason to exclude it and it only makes the code more
complicated and less consistent. Having it in the list provides an
easy way to detect if a package uses operations from package unsafe.

Change-Id: I2f9b0485db0a680bd82f3b93a350b048db3f7701
Reviewed-on: https://go-review.googlesource.com/37694
Reviewed-by: Alan Donovan <adonovan@google.com>