Package generation refactoring, stage 1.

Notable changes:

 - changelogs are now stored in XML (single source of information,
   ability to quickly extend for possible future export formats);

 - every module now has its own Makefile (easier maintenance,
   flexibility for fine-tuning of specific build pipelines);

 - introduced MODULE_TARGET environment variable for building
   the same set of modules for nginx/foss and NGINX Plus from
   the same set of sources.