Refactor support for orcidio as a new type of module.

[gouttegd]

(This PR is built on top on #23.)

Now that we can exclude individual import modules from the merged module, we no longer need to treat ORCIDIO as a completely ad-hoc thing, that is neither an import nor a component. Instead, we can treat merely as a type of import module.

That is, instead of

orcidio_support: true

the configuration should now be

import_group:
  products:
    - id: orcidio

We automatically recognise orcidio as a module of type orcidio, which can then be treated as any other import module – allowing us to remove most of the orcidio-specific code everywhere: all we need to keep is the custom rule to create the import module itself.

Just in case someone would want to import a module called orcidio that is not, in fact, the ORCIDIO from https://w3id.org/orcidio/, we don't do anything special if the module is declared with an explicit import type. For example:

import_group:
  products:
    - id: orcidio
      module_type: slme

This would be treated as a standard SLME module from an hypothetical orcidio ontology (which would here be expected to be located at http://purl.obolibrary.org/obo/orcidio.owl, since there's no mirror_from).

When base merging is enabled, the ORCIDIO module is automatically excluded from the merged module, since it requires its own extraction method.