Fix dropin name of main package when %mvn_package is not called

For simple or single feature plugins, where I want all IUs installed into the main package, I shouldn't have to specify this with a call to the "%mvn_package" macro.

If you do not make a call to "%mvn_package" then all IUs get installed into "/usr/share/eclipse/dropins" instead of the intended "/usr/share/eclipse/dropins/<dropin_name>"

This pull request fixes this simple use-case. If a target package is not specified (i.e. install into main package) then install into a dropin named after the common sub-package id.

This prevents IUs from being installed directly into the dropins directory instead of into an appropriately named directory under the dropins directory and prevents the plug-in package from accidentally owning the "/usr/share/eclipse/dropins" directory which is owned by eclipse-platform.