Go library: cannot find module providing package lib - go

Question

I am creating 3 separate go projects: ace, aces-client and a library shared by both projects aceslib. I am having trouble including the shared library according to the go-documentation (https://golang.org/doc/code.html#Library)
all go files in aceslib share the package-name aceslib. I am including the library in ace and aces-client with import lib "aceslib". I can build the library with go build and it gets installed with go install, in the directory listing one can see that the file go/pkg/windows_amd64/aceslib.a gets created.
But when I try to build ace or ace-client go complains:
$ go build
build ace: cannot load aceslib: cannot find module providing package aceslib
My go setup:
$ go version
go version go1.12.9 windows/amd64
$ go env
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\Yulivee\AppData\Local\go-build
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\Users\Yulivee\go
set GOPROXY=
set GORACE=
set GOROOT=c:\go
set GOTMPDIR=
set GOTOOLDIR=c:\go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=C:\Users\Yulivee\go\src\ace\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\Yulivee\AppData\Local\Temp\go-build024649150=/tmp/go-build -gno-record-gcc-switches
Directory Structure:
.
├── bin
│   ├── ace-client.exe
│   └── ace.exe
├── pkg
│   ├── mod
│   │   ├── cache
│   │   └── golang.org
│   └── windows_amd64
│   ├── ace
│   ├── aceslib.a
│   └── golang.org
└── src
├── ace
│   ├── README.md
│   ├── go.mod
│   ├── go.sum
│   └── main.go
├── ace-client
│   └── main.go
├── aceslib
│   ├── README.md
│   ├── crypto.go
│   ├── go.mod
│   └── utils.go
What am I missing?
Things I have tried, that did not help and lead to the same error:
$ go clean -i -x -cache -modcache
$ chmod 755 go/pkg/windows_amd64/aceslib.a
$ go get
build ace: cannot load aceslib: cannot find module providing package aceslib
$ rm -rf go/pkg/*

Answer 1

The directory structure and package names are a bit off - it should be a URL to your project's repo, like every other import that isn't from the stdlib. If you have no repo and want to use local references you'll need to add a replace directive to ace/go.mod like so:
replace aceslib => ../aceslib
As documented in the Modules docs.

Answer 2

IIRC, package names without directories are reserved for stdlib packages. So, you should first push the source tree down at least a level so you can access it by "dir/package".
Next, do you really want to treat these packages as separate modules? You have separate mod files under ace and aceslib packages. You may combine them under one module with a single go.mod at the project root and make life easier for yourself. That way you don't need redirects, etc.
You also have to include the modules with their directories:
import lib "dir/aceslib"