Refactor extract ebean-ddlgen module. In future this module would expected to be a test only dependency

[rbygrave]

Moves the DDL generation for both "drop-all/create-all" and DB migration generation into it's own module. Initially this will be included as a dependency of ebean (our top level composite) but in a future release this will instead ONLY be a dependency of ebean-test - so only a test scope dependency.

This is because we typically only need to generate DDL for testing purposes and for to generate DB migrations (which is also typically test scope).

Applications wanting to use DDL generation at runtime (in a deployed application) would then explicitly include this dependency as a compile scope dependency.