[build] Add java package in Maven build

[lbovet]

As alternative to #4002. This moves the generated classes into a Java package (and not keep them in the unnamed package) without disturbing the trgen process. This way, one can build a reusable JAR containing the lexer and parser.

The plugin configuration is done globally in the root POM. To benefit from it, grammar sub-projects have to adapt their POM and base classes. This PR includes the adaptation for the plsql grammar to illustrate what has to be done.

[lbovet]

Yes, this is an intermediate solution until there is something better integrated with the multi-target approach.

[teverett]

@lbovet thanks!