cmd/go: clarify error message when importing a package that could be (but isn't) in the main module

[johnrichardrinehart]

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

$ go version
go version go1.13 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

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

go env

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/johnrinehart/Library/Caches/go-build"
GOENV="/Users/johnrinehart/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/johnrinehart/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/johnrinehart/MyGo/playground/test/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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/wg/8x33rs4j5d7bgr5z58_4ql0m0000gn/T/go-build825724138=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I imported a path that was not a valid package (because of a combination of build tags, test files, documentation package files, and/or no files present, but was a valid path to a directory on a filesystem (one which contained (an)other package(s)). go build ./... and go test ./... and go mod tidy report an error in finding the path to the package within the module.

What did you expect to see?

I guess it would have been nice to see (both) 2 things output (as @Helcaraxan and @thepudds have suggested in Gophers slack (#modules):

1. There’s no such module online
2. No *.go files matching current build constraints

What did you see instead?

Before the module was pushed to a GitHub repository (everything local)

https://gist.github.com/johnrichardrinehart/2d9944249e4d1e0f17c608149d50ff39

and after the module was pushed to a repository (github.com/johnrichardrinehart/a) instead of (github.com/a, as in the gist):

johnrinehart@modie test (master) $ go mod tidy
go: finding github.com/johnrichardrinehart/a/folder latest
github.com/johnrichardrinehart/a/folder/pkg imports
        github.com/johnrichardrinehart/a/folder: no matching versions for query "latest"

[bcmills]

Comparable error messages are here and here.

Implementation-wise, this should probably be encoded as a special case of PackageNotInModuleError for Mod == Target, with a corresponding special case in its Error method.

CC @jayconrod