refactor(db)!: remove SourceObject.iter_virtual_sources from our API

BREAKING CHANGE: though a search of Lektor plugins on GitHub and PyPI
did not find any instance where a non-empty `iter_virtual_sources` was
defined

lektor/db.py

@@ -1513,8 +1513,8 @@ def track_record_dependency(self, record):
                 else:
                     affects_url = True
                 ctx.record_dependency(filename, affects_url=affects_url)
-            for virtual_source in record.iter_virtual_sources():
-                ctx.record_virtual_dependency(virtual_source)
+            if isinstance(record, VirtualSourceObject):
+                ctx.record_virtual_dependency(record)
             if getattr(record, "datamodel", None) and record.datamodel.filename:
                 ctx.record_dependency(record.datamodel.filename)
                 for dep_model in self.iter_dependent_models(record.datamodel):

lektor/sourceobj.py

@@ -47,10 +47,6 @@ def iter_source_filenames(self):
         if fn is not None:
             yield self.source_filename
 
-    def iter_virtual_sources(self):
-        # pylint: disable=no-self-use
-        return []
-
     @property
     def url_path(self):
         """The URL path of this source object if available."""
@@ -237,6 +233,3 @@ def alt(self):
     @property
     def source_filename(self):
         return self.record.source_filename
-
-    def iter_virtual_sources(self):
-        yield self