[MDEP-895] - dependency:sources fails for multi-module project

[jmle]

As described in MDEP-895, when running the sources goal against a multi-module project, the plugin is expecting all submodules to have been previously compiled. This shouldn't be necessary - even if the project in question generated its own sources, they shouldn't be needed, since they are included in the project.

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[ascheman]

Thanks, @slawekjaranowski

[michael-o]

This PR will be reverted with #408.