jsduck: Fix script loading issues with eg-iframe in production

It works locally because it can just nagivate to ../lib/ve.

In production however we need docs/ to be standalone so that we
can publish it without needing a subdirectory at:
https://docs.wikimedia.org/VisualEditor/master/.

For local usage this will be a symlink that still points to
the sibling directory (but using a symlink instead of a ../ path).
In production we can then instruct jenkins to replace that symlink
with an actual copy of lib/. As long as the generated path
references in the HTML remain the same.

Follows-up 88c488887214, which broke the publication of the docs in
Jenkins as it changed references from modules/ to ../modules/ for
the eg-iframe. This works locally, but broke in Jenkins as it was relying
on the copying of modules/ to docs/modules/ being enough to make it work).

Change-Id: I10eaa5424d172932b29774a0f03d511d555fd121