[macos] Go install fails with error: no install location for directory xxx outside GOPATH

You'll want to have 3 directories inside your chosen GOPATH directory.

GOPATH
     /bin
     /src
       /someProgram
        program.go
       /someLibrary
        library.go
     /pkg

Then you'll run go install from inside either someProgram (which puts an executable in bin) or someLibrary (which puts a library in pkg).