Before this change loading of a packerconfig would display `loaded
plugin ...` for any plugin defined in the packerconfig - even if it
didn't exist on disk. This change updates how plugins defined in
packerconfig are loaded to ensure that only successfully loaded plugins
get the lovely message `loaded plugin ...`. For any plugin that fails to
load Packer will log that it failed to load the plugin and continue
processing any other defined plugins.

* Add a test to ensure loadSingleComponent errors when plugin does not
exists

This change introduces a loadExternalComponent which can be used for
loading a single plugin path. The function is a combination of
the discoverSingle and discoverExternalComponents functions.