all: recognize new error from go command when no go.mod is found

This change fixes x/tools tests before CL 298650 lands. After that CL,
the go command reports a different error message when GO111MODULE=on
and it is invoked without a go.mod file (with a command that requires
a go.mod file). We plan to backport that change to 1.16.

For golang/go#44745

Change-Id: Idb3e146828703c89f788ae660ffc95aef16433e4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/298792
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Peter Weinberger <pjw@google.com>

internal/imports/mod.go

@@ -89,8 +89,10 @@ func (r *ModuleResolver) init() error {
 		err := r.initAllMods()
 		// We expect an error when running outside of a module with
 		// GO111MODULE=on. Other errors are fatal.
-		if err != nil && !strings.Contains(err.Error(), "working directory is not part of a module") {
-			return err
+		if err != nil {
+			if errMsg := err.Error(); !strings.Contains(errMsg, "working directory is not part of a module") && !strings.Contains(errMsg, "go.mod file not found") {
+				return err
+			}
 		}
 	}