Output not just a list of packages with headers, but their mapping to shlibs

We need to know not only which packages contain .h files, but also how they map to the shlib dependencies generated by the associated library packages because sometimes these refer to virtual packages and not the names of real package names built from the source.

This requires downloading all of the library packages to inspect them for a shlibs control file, there's no shortcut for this using index metadata.

As a side effect, this lets us also filter out from the analysis source packages that ship both .h files and .so files but do not ship shlibs for any .so files (IOW nothing links against those .so files).