Breadcrumbs are broken on odoc pages

[dbuenzli]

There are three problems on odoc pages:

1. The direct link to the package page jumps to the toplevel package list page rather than package page. See for example the odoc link here.
2. On non index.mld pages, the up link jumps to the toplevel package list rather than package page. See for example the up link here. It should jump to package page.
3. The breadcrumbs for index.mld should be specialized. I think best would be to have
   simply Up - PKG rather Up - LINKEDPKG >> index as it is now (see for example here). That is we should simply have an Up link going to the toplevel package list page, followed by the unlinked package name (because that's the page we are on).