Provide an option to download released packages when building plugins

[JustinKyleJames]

We often have to release a new version of a plugin that is tied to a released version of iRODS. Right now I believe that requires building the packages for core iRODS before building the plugin with the updates. Alternatively a person would have to manually download the packages and put them in the right directory before building the plugin.

It would be nice to have an option to retrieve the released iRODS packages during the plugin build.

[alanking]

Agreed. I think this needs to be implemented for each plugin build hook. This is an appropriate enough place to put the issue, though.