What version of Go are you using (go version)?

$ go version
1.12.4

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Ubuntu 16.04.6/amd64

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/root/.go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/root/.go/src/github.com/firebase/firebase-admin-go/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build012231754=/tmp/go-build -gno-record-gcc-switches"

What did you do?

The following is my tree:
snippets/
├── constants
│   └── constants.go
├── libs
│   ├── auth.go
│   ├── db.go
│   ├── init.go
│   ├── messaging.go
│   └── storage.go
└── snippets.go
I was trying to import constants from snippets.go:

cat snippets.go

package main
import (
	"snippets/constants"
)
.
.
.

And did:
go build -x -v

What did you expect to see?

No error or verbose output from go build

What did you see instead?

Quite unhelpful output:
build firebase.google.com/go/snippets: cannot load snippets/constants: cannot find module providing package snippets/constants

Now for whatever reason my import might have failed, I can understand that. But I would at least like a detailed explanation from go build. (In fact, the same is true to go get, go run etc.)