refactor: rename ModuleStore runtimes now that XModules are gone (#35523)

* 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.

docs/decisions/0020-upstream-downstream.rst

@@ -242,8 +242,7 @@ To support the Libraries Relaunch in Sumac:
     Video blocks.
 
 * We will define method(s) for syncing update on the XBlock runtime so that
-  they are available in the SplitModuleStore's XBlock Runtime
-  (CachingDescriptorSystem).
+  they are available in the SplitModuleStoreRuntime.
 
   * Either in the initial implementation or in a later implementation, it may
     make sense to declare abstract versions of the syncing method(s) higher up
@@ -355,10 +354,10 @@ inheritance hierarchy of CachingDescriptorSystem and SplitModuleStoreRuntime.)
 
 
     ###########################################################################
-    # xmodule/modulestore/split_mongo/caching_descriptor_system.py
+    # xmodule/modulestore/split_mongo/runtime.py
     ###########################################################################
 
-    class CachingDescriptorSystem(...):
+    class SplitModuleStoreRuntime(...):
 
         def validate_upstream_key(self, usage_key: UsageKey | str) -> UsageKey:
             """