x/vgo: cannot run vendor with any package that has appengine variants

[zombiezen]

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

go version go1.10.2 linux/amd64 vgo:2018-02-20.1

Does this issue reproduce with the latest release?

Yes, this occurs at vgo 48532fcbfed81df51d668e6d42180a115efc97ed. Issue introduced in 0b37e81747b0e67da0724ea17de3da7ad998cce0

What did you do?

$ cat > foo.go <<EOF
package main // import "example.com"

import (
        "fmt"

        _ "google.golang.org/appengine"
)

func main() {
        fmt.Println("Hello, World!")
}
EOF
$ touch go.mod
$ vgo vendor

What did you expect to see?

A materialized vendor directory.

What did you see instead?

vgo vendor fails with:

vgo: resolving import "google.golang.org/appengine"
vgo: finding google.golang.org/appengine (latest)
vgo: adding google.golang.org/appengine v1.0.0
vgo: resolving import "appengine"
vgo: import "appengine": unknown module appengine: not a domain name
vgo: resolving import "appengine_internal"
vgo: import "appengine_internal": unknown module appengine_internal: not a domain name
vgo: resolving import "appengine_internal/base"
vgo: import "appengine_internal/base": unknown module appengine_internal/base: not a domain name
vgo: resolving import "github.com/golang/protobuf/proto"
vgo: finding github.com/golang/protobuf (latest)
vgo: adding github.com/golang/protobuf v1.1.0
vgo: resolving import "golang.org/x/net/context"
vgo: finding golang.org/x/net (latest)
vgo: adding golang.org/x/net v0.0.0-20180611182652-db08ff08e862

/cc @rsc

[gopherbot]

Change https://golang.org/cl/118757 mentions this issue: cmd/go/internal/vgo: ignore appengine imports