export interfaces (build tool specific module definitions) from Export items

Export items make it possible to use Products as dependencies for other Products within a project. however, this data also needs to be exported for installed products, to make them available as external dependencies of other projects.

apart from qbs modules, we also need to export qt5 qmake module pri files (optional - needed only for qt modules), and cmake config files.

it must be possible to suppress this feature so internal modules are not exposed to the outside. note that this may depend on the build type - static builds may need to export convenience libraries which would be linked into the final dlls in shared builds.