I am new to Go - so this is not an expert advice. But I do it by creating multiple packages where more than one file belong to a particular package. You may already know that in Go lang if multiple files use the same package directive, the code across all the files are combined and treated as a giant code block. For example, if I am creating a blogging app then I'll have a package called adminpanel which comprises of files editor.go, publisher.go, moderation.go etc. So, a particular code in editor.go can access a function from publisher.go.
This is what I usually follow, but I would love to know what you guys use.