Warn on non-exported plugin packages

We want all plugin packages to be exported, to ensure other plugins can access classes from those packages. Some packages are meant to be kept internal. These packages can be exported as internal packages, still allowing them to be accessed. Uses of internal packages in other plugins will get flagged by the JDT compiler. The severity of these issues can be configured, and we currently have this set to warning level, I think. Accessing internal packages is sometimes useful when hacking something together as a test, or when something should not be internal, but changing that requires another release and developers working on code bases outside ESCET then want something that works now, such that they can continue, rather than having to wait a few months for the next release.

Currently, non-exported plugin packages don't give a warning. But PDT allows us to configure warnings for this. We could add that to our default set of Java configuration files, and apply that to all projects. If there is such a warning in the code base, then PDT allows addressing it with a quick fix, adding a 'normal' or 'internal' export.