_elementsources.py: Fix source names in unique key
The goal was to include the source plugin kind in the element cache key
as the unique key of a source may not be unique across different source
plugins. This is the source equivalent of the element-plugin-name
value in the element cache key.
However, Source._get_source_name()
was the wrong method for this as
that also includes the key itself, which may not even be set yet.
This results in a cache key change.
Fixes: 3953bcc6 ("element.py: clobber sources with workspace")