Starting Jetty with JPMS produces warnings about Servlet resources not found

**Jetty version**
10.0.x & 11.0.x

**Description**
Start Jetty with `--jpms` and you'll see a number of warning like the one below:

```
2021-01-21 16:18:10.049:WARN :oatud.DigesterFactory:main: The XML schema [XMLSchema.dtd] could not be found. This is very likely to break XML validation if XML validation is enabled.
```


