[ruby/prism] Rework Java template generation for Maven build

This uses the JRuby rake-maven-plugin to generate the templates
as part of the Maven build. The generated output for the Java
templates will be under java/api/target/generated-sources/java.

ruby/prism@b257151391

Author: headius

prism/templates/template.rb

@@ -688,9 +688,9 @@ def locals
       "javascript/src/deserialize.js",
       "javascript/src/nodes.js",
       "javascript/src/visitor.js",
-      "java/api/src/main/java/org/ruby_lang/prism/Loader.java",
-      "java/api/src/main/java/org/ruby_lang/prism/Nodes.java",
-      "java/api/src/main/java/org/ruby_lang/prism/AbstractNodeVisitor.java",
+      "java/api/target/generated-sources/java/org/ruby_lang/prism/Loader.java",
+      "java/api/target/generated-sources/java/org/ruby_lang/prism/Nodes.java",
+      "java/api/target/generated-sources/java/org/ruby_lang/prism/AbstractNodeVisitor.java",
       "lib/prism/compiler.rb",
       "lib/prism/dispatcher.rb",
       "lib/prism/dot_visitor.rb",