librustc::session : Make DebuggingOpts use the options! macro
librustc::metadata : Allow passing a string to read_plugin_metadata
librustc::plugin : make PluginLoader usable for loading argument-specified plugins
librustc_driver: Add support for loading plugins via command line (fixes #15446)
libsyntax: add COMMAND_LINE_SP and use it for spans generated from the command line
Add test for -Z extra-plugins