Several macOS-specific modules do expensive calls to calculate their properties

I observed that module loading is orders of magnitude slower on macOS compared to other platforms. Profiling showed that the validate() functions for xcode and ib are the culprit (they were slower by a factor of ~100 compared to other modules). A quick look at the xcode module shows that operations like File.directoryEntries() occur at the right-hand side of properties.
We must not do that. All potentially expensive operations, in particular those accessing the file system, need to be done in Probes, whose results are cached.