Respect relative paths in include-ed content

[fwkoch]

When you use the include directive to pull in contents from another file, content is processed relative to the base file (with the include directive). In the PR that added support for recursive includes #1082, we made sure recursive includes relative paths to be relative to the base file. However, it did not address image/embed/etc nodes in the included content which may also have relative paths.

We need to ensure any node type with a relative local path is updated to be relative to the base file when pulled in using an include directive.

[thesamovar]

+1 for this, I ran into this almost immediately when using the include directive.