Support multi-module projects

[gunnarmorling]

By design Deptective works at compile time of individual modules and the defining use case has been to ensure the architecture within single modules. But it's desirable to also enforce architectural rules across module boundaries (so e.g. the code of the module under compilation should only access certain packages from another module it depends on). This is already possible by simply expressing the right alowed package relationships.

There'll be redundancy though when adding "component" definitions into the picture (#28). Then it should be possible to refer to the components declared in module A when defining the architecture of module B depending on A ,e.g. B should only access components A1 and A2. For that, we could take into account component definitions in META-INF/deptective.json of the dependencies when compiling B. That way, B's deptective.json could refer to components declared in A.