Bug 21895: Fix translation for package install

Bug 21895: Fix translation for package install

The string extraction process was not taking into account the fact that
standard/package install have a completely different directory structure
than the dev install

This patch tries to keep the exact same behaviour for dev installs,
while making it work for standard install by using opachtdocs,
intrahtdocs, opacdir and intranetdir from $KOHA_CONF

Test plan:
1. Follow test plan in
https://gitlab.com/koha-community/Koha/commit/d708255c7a4d981c7c7bdd0644a75202ec43b297
2. Do a standard install and repeat step 1 on this new install
3. If you know how to build the Debian package, build it, install it and
verify that koha-translate works as expected
4. prove t/LangInstaller.t

Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>