go/build: setting UseAllFiles still invokes go command

[rsc]

In build.go:

func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode) error {
	// To invoke the go command,
	// we must not being doing special things like AllowBinary or IgnoreVendor,
	// and all the file system callbacks must be nil (we're meant to use the local file system).
	if mode&AllowBinary != 0 || mode&IgnoreVendor != 0 ||
		ctxt.JoinPath != nil || ctxt.SplitPathList != nil || ctxt.IsAbsPath != nil || ctxt.IsDir != nil || ctxt.HasSubdir != nil || ctxt.ReadDir != nil || ctxt.OpenFile != nil || !equal(ctxt.ToolTags, defaultToolTags) || !equal(ctxt.ReleaseTags, defaultReleaseTags) {
		return errNoModules
	}

ctxt.UseAllFiles == true is special and should be in the list.

/cc @FiloSottile