Maven AOT plugin injects class files into sources jar

The way that the AOT classes and metadata are generated means they end up in the sources jar of a Maven project. They should not be there.

Example from the Prism parser for JRuby:

```
$ jar tf wasm/target/prism-parser-wasm-0.0.2-SNAPSHOT-sources.jar
META-INF/
META-INF/MANIFEST.MF
org/
org/ruby_lang/
org/ruby_lang/prism/
org/ruby_lang/prism/wasm/
META-INF/maven/
META-INF/maven/org.ruby-lang/
META-INF/maven/org.ruby-lang/prism-parser-wasm/
org/ruby_lang/prism/wasm/Prism.java
org/ruby_lang/prism/wasm/WasmResource.java
org/ruby_lang/prism/wasm/PrismParser.java
org/ruby_lang/prism/wasm/Prism_WasiSnapshotPreview1.java
org/ruby_lang/prism/wasm/Prism_ModuleExports.java
org/ruby_lang/prism/wasm/Prism_ModuleImports.java
org/ruby_lang/prism/wasm/PrismParser.meta
org/ruby_lang/prism/wasm/PrismParserMachineFuncGroup_0.class
org/ruby_lang/prism/wasm/PrismParserMachineDispatch_0.class
org/ruby_lang/prism/wasm/PrismParserMachineShaded.class
org/ruby_lang/prism/wasm/PrismParserMachineMachineCall.class
org/ruby_lang/prism/wasm/PrismParserMachine.class
META-INF/maven/org.ruby-lang/prism-parser-wasm/pom.xml
META-INF/maven/org.ruby-lang/prism-parser-wasm/pom.properties
```

I tried using `targetClassesDirectory` to put them somewhere else but could not figure out how to make the build correctly see them for the rest of compilation.