all: import "context" instead of "golang.org/x/net/context"

There is no need to depend on the old context package now that the
minimum Go version is 1.7. The move to "context" eliminates our weird
vendoring setup. Some vendored code still uses golang.org/x/net/context
and it is now vendored in the normal way.

This change triggered new vet checks around context.WithTimeout which
didn't fire with golang.org/x/net/context.

internal/build/util.go

@@ -52,19 +52,10 @@ func MustRunCommand(cmd string, args ...string) {
 // GOPATH returns the value that the GOPATH environment
 // variable should be set to.
 func GOPATH() string {
-	path := filepath.SplitList(os.Getenv("GOPATH"))
-	if len(path) == 0 {
+	if os.Getenv("GOPATH") == "" {
 		log.Fatal("GOPATH is not set")
 	}
-	// Ensure that our internal vendor folder is on GOPATH
-	vendor, _ := filepath.Abs(filepath.Join("build", "_vendor"))
-	for _, dir := range path {
-		if dir == vendor {
-			return strings.Join(path, string(filepath.ListSeparator))
-		}
-	}
-	newpath := append(path[:1], append([]string{vendor}, path[1:]...)...)
-	return strings.Join(newpath, string(filepath.ListSeparator))
+	return os.Getenv("GOPATH")
 }
 
 // VERSION returns the content of the VERSION file.