Support multi-shot providers

Currently, provider is invoked only once generating multiple modules (not the case for the fallback provider though) at once.
Given the fact we now support Probes in the Provider which allow to cache expensive calls to external tools (e.g. qmake or pkgconfig), it now might make sense to invoke provider every time for a module.
In that case, we can support providing one module via other providers (e.g. Qt from qmake and QtScript via Manual provider or pkgconfig).
One thing to think about - should we include module name in a hash (which will result to a different searchPath for each module), or should we exclude it and write all modules in the same dir? If dir is different the pros is that we should not handle name prop in a special way when generating hash; the cons is that modules are scattered all over the place and it's hard to find them. The problem does not exist if QBS-1707 is implemented.