cmd/go/internal/vgo: vgo does not support relative imports

Fixes golang/go#24157.

Change-Id: I6b34a8c3a3bd64db54041a1e257ae9714cb1be98
Reviewed-on: https://go-review.googlesource.com/108378
Reviewed-by: Russ Cox <rsc@golang.org>

vendor/cmd/go/internal/vgo/load.go

diff --git a/vendor/cmd/go/internal/vgo/load.go b/vendor/cmd/go/internal/vgo/load.go
index ea4095b..fb508de 100644
--- a/vendor/cmd/go/internal/vgo/load.go
+++ b/vendor/cmd/go/internal/vgo/load.go
@@ -8,6 +8,7 @@
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"go/build"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -305,6 +306,10 @@
 			return
 		}
 	}
+	if build.IsLocalImport(m.path) {
+		base.Errorf("vgo: relative import is not supported: %s", m.path)
+		return
+	}
 	fmt.Fprintf(os.Stderr, "vgo: resolving import %q\n", m.path)
 	repo, info, err := modfetch.Import(m.path, allowed)
 	if err != nil {