cmd/go: mod why -m is not strict when parsing an import path

[perillo]

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

$ go version
go version go1.13.6 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/home/manlio/.local/bin"
GOCACHE="/home/manlio/.cache/go-build"
GOENV="/home/manlio/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB="github.com/perillo/*"
GOOS="linux"
GOPATH="/home/manlio/.local/lib/go:/home/manlio/src/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/manlio/src/go/src/github.com/perillo/database/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-build853843883=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.13.6 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.13.6
uname -sr: Linux 5.4.12-arch1-1
/usr/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.30.
gdb --version: GNU gdb (GDB) 8.3.1

What did you do?

In a project that depends on golang.org/x/tools/go/packages, I executed

go mod why -m golang.org/x/tools/ 

What did you expect to see?

go: malformed import path "golang.org/x/tools/": trailing slash

as it is done by go mod init.

What did you see instead?

# golang.org/x/tools/
(main module does not need module golang.org/x/tools/)

[gopherbot]

Change https://golang.org/cl/216097 mentions this issue: cmd/go: validate module path in mod why