Consider reorganizing metadata import code

[landreev]

As currently implemented, metadata import for the supported formats (DC, DDI...) is hard-coded in the application. We should consider changing it to something similar to how the exports are implemented (as Exporter classes that ExportService loads dynamically). This would make it easier to add support for more formats (and, potentially, for harvesting from more sources).

If we wanted to be really fancy, we could add support for loading these as real dynamic plugins, that could be dropped in place, without needing to add the code to the application project).