HeaderSearch::LookupFile() looks for the file in the includers' dirs
first. But it doesn't bother to check whether a module exists for it.
Attached patch fixes that - but in a clumsy way; I'd prefer to reduce
the code duplication between DirectoryLookup::LookupFile() and
HeaderSearch::LookupFile() rather than increasing it.
The attached test tests this - sadly also in a clumsy way Please