Reorganize explorer packages, make service pluggable

The apidoc* modules are currently done in the following way:

• nuxeo-apidoc-core holding most of the code and logics
• nuxeo-apidoc-repo holding more repsitory-related logics as well as JSF dependencies
• nuxeo-apidoc-webengine holding the webengine pages to download/upload/export a zipped runtime distribution, as well as JSF dependencies

There are 2 packages:

• one remnant that should be removed at https://github.com/nuxeo/nuxeo/tree/master/modules/platform/nuxeo-apidoc-server/pkg
• the legacy one that should be removed at https://github.com/nuxeo/marketplace-platform-explorer (as well as dedicated marketplace.ini mention)

After the reorganization, we would like 3 packages, with 4 modules:

• nuxeo-apidoc-core holding the interfaces and everything needed to snapshot the current live runtime (with corresponding package, independent of UI, to serve need at NXP-27952)
• nuxeo-apidoc-repo holding the persistence logics as Nuxeo documents and corresponding XML export features
• nuxeo-apidoc-webengine holding the webengine pages to explore live repository as well as persisted one (to be put back in nuxeo main repository instead of the JSF package)
• nuxeo-apidoc-jsf holding the JSF dependencies, for JSF display of documents but also export of JSF-specific configurations (typically seam components) – this extract will probably require more pluggable points in the apidoc-core module logics

Name of corresponding packages:

• legacy platform-explorer package will be kept, holding everything except the JSF dependencies
  • TODO: check if conflicts are still valid (see NXP-27856), at least nuxeo-dam and nuxeo-social-collaboration have been deprecated for a long time. nuxeo-quota might be marked as conflicting because it's annoying for export features (to be checked in tests)
• a new platform-explorer-jsf package will be added to keep the JSF exports for now

Later, we'll see if extracting a dedicated rest-api is useful, without the "repo" dependencies.

NB: with the nex extension point, it appeared that json export was broken and not used anywhere else than in tests: this work includes a fix for this export (without keeping compatibility in the format) to make it usable for further work with NXP-28838, only on "live" distributions (json export on persisted distributions will not be implemented here).