go version go1.10.1 linux/386 vgo:2018-02-20.1

GOARCH="386"
GOBIN=""
GOCACHE="/home/mjohn/.cache/go-build"
GOEXE=""
GOHOSTARCH="386"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/mjohn/workspaces/go"
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_386"
GCCGO="gccgo"
GO386="sse2"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m32 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -gno-record-gcc-switches"
VGOMODROOT=""

.
├── A
│   ├── a.go
│   └── go.mod
└── cmd
    ├── B
    │   ├── go.mod
    │   └── main.go
    └── C
        ├── go.mod
        └── main.go

❯❯❯ cat A/a.go
package "foo/A"

import "fmt"

func SayHello() {
	fmt.Println("Hello")
}

func SayBye() {
	fmt.Println("Bye")
}
❯❯❯ cat cmd/B/main.go
package main // import "foo/B"

import "foo/A"

func main() {
	A.SayHello()
}
❯❯❯ cat cmd/C/main.go
package main // import "foo/C"

import "foo/A"

func main() {
	A.SayBye()
}
❯❯❯ cat A/go.mod
module "foo/A"
❯❯❯ cat cmd/B/go.mod
module "foo/B"
❯❯❯ cat cmd/C/go.mod
module "foo/C"

❯❯❯ vgo build cmd/B/main.go
vgo: cannot determine module path for legacy source directory /home/mjohn/workspaces/foo (outside GOROOT, no import comments)