Internal path links are smart

When a link is an internal path link this has the following effects:

the correct relative path to the file is used when using nested scopes with -S, --split-headers. For example, if we have:
```
= h1

== h2
{scope}

=== h3

\a[index.js]
```
then in split header mode, h3 will be rendered to h2/h3.html.
Therefore, if we didn't do anything about it, the link to index.js would render as href="index.js" and thus point to h2/index.js instead of the correct index.js.
Instead, OurBigBook automatically converts it to the correct href="../index.js"
the _raw directory prefix is added to the link
existence of the file is checked on compilation. If it does not exist, an error is given.

 Ancestors (7)