Move Report builder classes into /classes/reportbuilder/ namespace

This issue is a follow-up to Jake's review on https://tracker.moodle.org/browse/MDL-74364.

This is about moving Report builder classes into /classes/reportbuilder/ namespace and adding debugging calls to highlight cases where the entities (and any other reportbuilder-specific classes) are not located under the 'reportbuilder' level 2 namespace.

"This will allow us to catch cases like this without needing to manually check during reviews. At a glance, I think add_entity() is a candidate for this, but we may also wish to consider ways to ensure specific reportbuilder namespaces are matched in the respective plugin implementations too. E.g. reportbuilder\systemreport, reportbuilder\entities, reportbuilder\audiences to name a few that are present in the plugins below. You'll note that some of these are in the PLUGINNAME\reportbuilder namespace while others are not.

Other components which don't use the level 2 namespace for all reportbuilder classes (candidates for future update):

• admin
• cohort
• report_configlog"