cmd/go: `go get -u=patch golang.org/x/tools` broken at head

[bcmills]

The fix for #26902 seems to have broken -u=patch when the command-line argument is a module that does not contain a package at its root.

$ gotip version
go version devel +323212b9 Sun Jun 9 16:23:11 2019 +0000 linux/amd64

$ mkdir example.com

$ cd example.com

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

example.com$ gotip get -u=patch golang.org/x/tools
go: finding golang.org/x/tools latest
go: downloading golang.org/x/tools v0.0.0-20190610194359-fe937a7521e5
go: extracting golang.org/x/tools v0.0.0-20190610194359-fe937a7521e5
go get golang.org/x/tools: module golang.org/x/tools@latest (v0.0.0-20190610194359-fe937a7521e5) found, but does not contain package golang.org/x/tools

CC @jayconrod @jba

[jayconrod]

I believe CL 180337 should fix this. I'll verify and update the commit message.

[gopherbot]

Change https://golang.org/cl/180337 mentions this issue: cmd/go: avoid accidental downgrades in 'go get' with latest and patch