Add support for .implib on Windows

Imported libraries are collected as object files, because they need to be found in Clean System Files in the same way. In the link step (add_imported_object_files) we check if the dependency is a normal object file or a library, and prepend -l if it is a library.

Library names starting with -l are still ignored so that you can use import code from library "-lmysql" and import code from library "mysql_library" in the same implementation module and compile for POSIX and Windows.