go/ast/astutil: add merging pass to AddImport and AddNamedImport.

After inserting an import, merge all import declarations.

Fixes golang/go#14075.

Change-Id: I17fceb60f490deced2ee8eadf78091720580ffa1
Reviewed-on: https://go-review.googlesource.com/19828
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed