(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.

As result, this means that the plugin packager will now either package
everything in $plugin_name/lib/mcollective/ or $plugin_name/, depending
on the chosen layout.
(maint) - Fix modulepackage packager when passed a path on the cli