Add support for .implib on Windows

Ghost User
2020-10-16T12:45:56+02:00

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.