Improve resolution of conflicting Symbols in JCasC

Currently JCasC classes resolve configurators by explicit or implied `@Symbol`, but they do not take the context into account the context (e.g. target type). If there are two symbols in different extension points, a wrong one might be picked. https://wiki.jenkins.io/display/JENKINS/Structs+plugin explicitly permits coflicting symbols in different contexts, so we should support it in JCasC somehow.

Use-cases:

* Target type / extension point
* Generic types within the same extension points. Gewneric can be used as additional source of context resolution