go module question

gistfile1.txt

Module structure:
~/dev/go/collect 
$ tree
.
├── go.mod
├── go.sum
├── LICENSE.txt
├── maps
│   ├── maps_gen.go
│   ├── maps.go
│   └── maps_test.go
├── README.md
├── sets
│   ├── sets.go
│   └── sets_test.go
└── slices
    ├── slices_gen.go
    ├── slices.go
    ├── slices_int.go
    └── slices_test.go
   

content of go.mod:
module github.com/apahl/collect

require (
        github.com/cheekybits/genny v1.0.0
        golang.org/x/crypto v0.0.0-20180910181607-0e37d006457b // indirect
        golang.org/x/net v0.0.0-20180921000356-2f5d2388922f // indirect
        golang.org/x/sys v0.0.0-20180920110915-d641721ec2de // indirect
        golang.org/x/text v0.3.0 // indirect
        golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e // indirect
)



The problem is: even after issuing `go install ...`, I can not import any of the packages from this module.
E.g. `import "github.com/apahl/collect/sets"` in another project fails.

What am I doing wrong here? Do I need to put go.mod files in all the sub-package dirs (maps, sets, slices) as well?