The plugin layouts are different across platforms. Move this into a
virtual method and allow replacement. On Windows, the plugins are
placed into the `bin` directory as the DLLs should always be co-located
to ensure that the proper DLLs are found (there is no concept of RPATH).
'load-plugin-library', 'load-plugin-executable', '-plugin-path' and
'-external-plugin-path' should be searched in the order they are
specified in the arguments.
Previously, for example '-plugin-path' used to precede
'-external-plugin-path' regardless of the position in the arguments.
