Understanding How to Build Go Source

I am confused about how to layout my Go project and then how to build it. I am reasonably new to Go and believe there was a time <1.13 where the GOPATH was important. I am using 1.14, so I believe I do not have to care about that and GOPATH is not set. I do not (for the moment) host my code on GitHub (which is something various articles assume). I have read a number of things, but it all leaves me more confused:

I have laid out my project according to this: https://github.com/golang-standards/project-layout. (Except this project seems to use a Makefile. I do not want to write a Makefile. I believe this should all work without a Makefile. I really do not want to write a Makefile.)

Here is the structure:

/src
  /cmd
    main.go
  /internal
    helper.go
  go.mod

go.mod looks like this:

module mycompany/mymodule
    
go 1.14

service.go looks as follows
package main // this has to be called ‘main’

import (
    "mycompany/mymodule/internal/helper"
)

func main () {
    helper.greet("Jenny")
}

So, if I am in /src and I run build cmd/service.go I get
cmd/service.go:4:2: package mycompany/mymodule/internal/helper is not in GOROOT (/usr/local/Cellar/go/1.14.5/libexec/src/mycompany/mymodule/internal/helper)

Do I have to compile helper.go first? Manually? Surely not. go build surely should be able to build my entire project, right? Including all the dependencies. Could someone please tell me what I am missing?

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