Allow exports to modules outside the (parent) search path

Our project uses code generated by a code generator that resides in its own subproject (in fact it is a separate Git submodule). The Qbs module with the Rule item that executes the generator is part of that subproject.

However, it is currently not possible to declare an exported dependency on a module that resides in a sub project without augmenting the qbsSearchPaths property of the super project appropriately (cf. tests/auto/blackbox/testdata/export-to-outside-searchpath).

It would be nice, if it was sufficient to declare a dependency on the code generator (a product in the subproject) in a product in the super project for the code generation to "just work".

I see at least two possible solutions:

1. implicitly augment the super projects qbsSearchPaths with the one from the subproject (although that might introduce a circular dependency, since the subproject's qbsSearchPaths gets already merged with the super project's one)
2. allow the Export item to augment the qbsSearchPaths property of the importing project