Resolving rule unit classnames depends on File.separator character

When running testNamingConventionOnDrlFile test on Windows, it fails on NPE, because the engine cannot find the rule unit "org/kie/test/MyRuleUnit" defined in the test, because it contains "/" separator character. The method that resolves the classname searches for File.separator character in the mentioned string, which is "\" on Windows [1].

This is a bug, because e.g. MemoryFileSystem uses "/" also when running on Windows. The classname resolution should be OS independent.

[1] https://github.com/kiegroup/drools/blob/5fad04919f255933bdd34afdb8da10b94f2c8c43/drools-compiler/src/main/java/org/drools/compiler/rule/builder/RuleBuildContext.java#L184