(MCO-314) mco plugin package should support 'lib layout

This commit enables the plugin packager to support plugins using the
'lib layout'. Since this layout also suggests using sub directories in
the main plugin directories (for example
mcollective-shell-agent/agent/shell/*) this commit also extends the
plugin content lookup to include subdirectories.

The modulepackage packager will now also create the correct packages
when a path is specified on the command line.
get_plugin_path as discussed, is probably more apt.