Repository: go
Issue: cmd/go: 'go get' should show requirement chains for inconsistent versions
Labels: NeedsFix,GoCommand,modules

<!-- Please answer these questions before submitting your issue. Thanks! -->

### What version of Go are you using (`go version`)?

<pre>
$ go version
go version go1.13.1 darwin/amd64
</pre>

### Does this issue reproduce with the latest release?

Yes.

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/deklerk/Library/Caches/go-build"
GOENV="/Users/deklerk/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/deklerk/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="/tmp/foo/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/c2/cvxltzcd66v5lx14hm1j76q000h16k/T/go-build418507423=/tmp/go-build -gno-record-gcc-switches -fno-common"
</pre></details>

### What did you do?

```
$ go get cloud.google.com/go/pubsub@v1.0.1-beta.ordered.keys
go: finding cloud.google.com v1.0.1-beta.ordered.keys
go: finding cloud.google.com/go v1.0.1-beta.ordered.keys
go get: inconsistent versions:
	cloud.google.com/go/pubsub@v1.0.1-beta.ordered.keys requires cloud.google.com/go/pubsub@v1.0.1 (not cloud.google.com/go/pubsub@v1.0.1-beta.ordered.keys)
$
```

### What did you expect to see?

It works.

### What did you see instead?

It does not work. I assume it's because there's a path from v1.0.1-beta to v1.0.1 (see below), but not sure why that's a problem.

```
deklerk at deklerk-macbookpro2 in ~/workspace/google-cloud-go/pubsub on pubsub-ordered-keys
$ go mod graph | grep pubsub
cloud.google.com/go/pubsub cloud.google.com/go@v0.46.3
cloud.google.com/go/pubsub cloud.google.com/go/storage@v1.0.0
cloud.google.com/go/pubsub github.com/golang/protobuf@v1.3.2
cloud.google.com/go/pubsub github.com/google/go-cmp@v0.3.0
cloud.google.com/go/pubsub github.com/googleapis/gax-go/v2@v2.0.5
cloud.google.com/go/pubsub go.opencensus.io@v0.22.0
cloud.google.com/go/pubsub golang.org/x/exp@v0.0.0-20190912063710-ac5d2bfcbfe0
cloud.google.com/go/pubsub golang.org/x/oauth2@v0.0.0-20190604053449-0f29369cfe45
cloud.google.com/go/pubsub golang.org/x/sync@v0.0.0-20190423024810-112230192c58
cloud.google.com/go/pubsub golang.org/x/time@v0.0.0-20190308202827-9d24e82272b4
cloud.google.com/go/pubsub golang.org/x/tools@v0.0.0-20190917162342-3b4f30a44f3b
cloud.google.com/go/pubsub google.golang.org/api@v0.9.0
cloud.google.com/go/pubsub google.golang.org/genproto@v0.0.0-20190911173649-1774047e7e51
cloud.google.com/go/pubsub google.golang.org/grpc@v1.21.1
cloud.google.com/go/pubsub@v1.0.1 cloud.google.com/go@v0.45.1
cloud.google.com/go/pubsub@v1.0.1 google.golang.org/grpc@v1.21.1
cloud.google.com/go/pubsub@v1.0.1 google.golang.org/genproto@v0.0.0-20190819201941-24fa4b261c55
cloud.google.com/go/pubsub@v1.0.1 google.golang.org/api@v0.9.0
cloud.google.com/go/pubsub@v1.0.1 golang.org/x/time@v0.0.0-20190308202827-9d24e82272b4
cloud.google.com/go/pubsub@v1.0.1 golang.org/x/sync@v0.0.0-20190423024810-112230192c58
cloud.google.com/go/pubsub@v1.0.1 golang.org/x/oauth2@v0.0.0-20190604053449-0f29369cfe45
cloud.google.com/go/pubsub@v1.0.1 go.opencensus.io@v0.22.0
cloud.google.com/go/pubsub@v1.0.1 github.com/googleapis/gax-go/v2@v2.0.5
cloud.google.com/go/pubsub@v1.0.1 github.com/google/go-cmp@v0.3.0
cloud.google.com/go/pubsub@v1.0.1 github.com/golang/protobuf@v1.3.2
cloud.google.com/go@v0.46.3 cloud.google.com/go/pubsub@v1.0.1
```