Minor complication with PlantUML diagrams in a sub-site

[damithc]

Scenario:
Assume a sub-site has a puml diagram.
When you build the outer site, there is no problem.
But if you go and build the sub-site, and then try to build the outer-site, there will be a file not found error. The error goes away if you delete the _site folder of the sub-site before building the outer site.

[crphang]

I would check if #968 could resolve this as well. My approach would be moving the logic of PlantUML generation away from plugins for both inline and filebased. The intention is to:

1. Unify code logic
2. Less side effects from DOM manipulation which in this case could be the reason.