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.
1 job for implib-support-for-windows in 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #109890
linux

00:00:14