Make src/ directory the general root namespace.

This is much handier for when new code/namespaces are introduced because
one does not need to add them to the composer.json file anymore.
Merge branch 'master' into multisite
Add site ID to the invalidation of the cache.

Because invalidation of the file cache needs to be site dependent.
Note: the regex refactoring that was needed for this is now supported by
a unit test.