feat:Support for referencing types in different API files using format

[chensylz]

maybe I just want to format api files

[chensylz]

now I have this case

-root
- apis
- a.api
- b.api
main.api

main.api use
import "apis/a.api"
import "apis/b.api"

a.api use b.api type
such as
b.api
type A {}
a.api
type B { AList []A 'json:"a_list"' }

now, goctl api go --api /root/main.api functioning
but, goctl api format --dir /root don't work, raise can not found declaration 'A' in context

I just want to format api files, don't need check declare, because goctl api go will be help me out

[kevwan]

lgtm.