cmd/go: 'go get github.com/Azure/[email protected]' silently gets 'v20.1.0' instead

[bcmills]

Found while investigating #27457.

$ cd $(mktemp -d)

$ go mod init example.com
go: creating new go.mod: module example.com

$ go get github.com/Azure/[email protected]

$ go list -m all
example.com
github.com/Azure/azure-sdk-for-go v20.1.0+incompatible

$ go version
go version go1.11 linux/amd64

$

[thepudds]

@bcmills I don’t know if you’ve had a chance to look at this much yet, but do you have an initial guess or sense of what might be going on here, or maybe a guess around possible correlating factors, etc.?

[bcmills]

The underlying cause here is the same as #29121: if the requested package isn't found in the requested version, we're falling back to latest instead of emitting an error as we should.