* Consolidates and renames the runtime used as a base for all the others:
* Before: `xmodule.x_module:DescriptorSystem` and
`xmodule.mako_block:MakoDescriptorSystem`.
* After: `xmodule.x_module:ModuleStoreRuntime`.
* Co-locates and renames the runtimes for importing course OLX:
* Before: `xmodule.x_module:XMLParsingSystem` and
`xmodule.modulestore.xml:ImportSystem`.
* After: `xmodule.modulestore.xml:XMLParsingModuleStoreRuntime` and
`xmodule.modulestore.xml:XMLImportingModuleStoreRuntime`.
* Note: I would have liked to consolidate these, but it would have
involved nontrivial test refactoring.
* Renames the stub Old Mongo runtime:
* Before: `xmodule.modulestore.mongo.base:CachingDescriptorSystem`.
* After: `xmodule.modulestore.mongo.base:OldModuleStoreRuntime`.
* Renames the Split Mongo runtime, the which is what runs courses in LMS and CMS:
* Before: `xmodule.modulestore.split_mongo.caching_descriptor_system:CachingDescriptorSystem`.
* After: `xmodule.modulestore.split_mongo.runtime:SplitModuleStoreRuntime`.
* Renames some of the dummy runtimes used only in unit tests.
This re-applies commit 36cc415 with handling an invalid context_key in the
`PartitionService`. It can happen when rendering a `LibraryContentBlock` in
Studio because this service is initialized by the modulestore when validating
an XBlock to gather its error messages in the `studio_xblock_wrapper`.
