在Go中的同一项目中导入文件的正确方法

I have a Go project with a structure like this:

cmd
  myapp
    main.go
internal
  app
    myapp
      impl.go

Now in main.go, I want to import impl.go. GoLand tells me that I should do it like this:

import (
  "myproj/internal/app/myapp"
)

However, I compile the project on another machine (because I use EGL and that's not available on macOS). When I do

go build cmd/myapp/main.go

there, it cannot resolve the import. So I changed the import to use the full path of my github repo, like this:

import (
  "github.com/flyx/myproj/internal/app/myapp"
)

Code compiles fine now. However, after I pulled that back into GoLand, it cannot resolve the import anymore. Curiously, every item in the path gives a separate error, e.g.

Cannot resolve directory 'github.com'

That's very strange since github.com is very obviously in my $GOPATH/src and other imports from there that are not my current project are working.

So… what am I doing wrong? I don't even understand how GoLand can resolve the path starting with myproj. I checked whether GOPATH is correctly set in GoLand and that's the case.