x/vgo: get fails if there are no go files at the module root

[zombiezen]

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

$ vgo version
go version go1.10 darwin/amd64 vgo:2018-02-20.1

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/light/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/light"
GORACE=""
GOROOT="/Users/light/go"
GOTMPDIR=""
GOTOOLDIR="/Users/light/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
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/4p/yyp29g691d17jrvrwq2gzxxr004_bp/T/go-build301961857=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

$ mkdir foo
$ cd foo
$ echo 'module "myroot"' > go.mod
$ vgo get github.com/googleapis/gax-go
vgo: import "myroot" [/Users/light/foo]: no Go source files

What did you expect to see?

vgo to exit successfully, with the following content in go.mod:

module "myroot"

require "github.com/googleapis/gax-go" v2.0.0

What did you see instead?

vgo exits with failure and has the following content in go.mod:

module "myroot"

[rsc]

Related to #24105.

[gopherbot]

Change https://golang.org/cl/120995 mentions this issue: cmd/go/internal/vgo: allow vgo get of module with no package in root