Fix packaging of the backend module

[sekruse]

Currently the build fails because the backend module is packaged as war while at the same time being referenced by other modules (e.g., example_sql_profiling_algorithm) that require jar packaging.

A solution might be to have separate modules for the code and packaging (e.g., with a distro module). Such an effort could also make it easier to build a "headless" Metanome, that is operated from the command line or used as a library in third-party applications.

[thorsten-papenbrock]

Fixed: We now build both jar and war in two separate modules.