"Block library" templates have blank lines marked as missing

Some libraries use blocks from templates that have no {% extends %} tag. Normally a template like that would be fully rendered, so the blank lines between the blocks would be rendered. Since these templates are never rendered in full, those blank lines are never rendered. But coverage doesn't understand how these files are being used (as block libraries), so it marks the interstitial blank lines as missing.

Examples of libraries that do this: django-email-template, formulation and django-sniplates.

Start by examining how the Django coverage plugin identifies executed template lines for block libraries without an {% extends %} tag. Compare its behavior with the django-email-template, formulation, and django-sniplates examples; done means interstitial blank lines in these libraries are no longer reported as missing.