This allows modules that declare their class to be checked.
While that's not most user modules, frameworks can take advantage
of this by setting declaring the module class for their users.
That way, the mistake of importing a module into the wrong hierarchy
can be reported more clearly in some cases.
these files provided mainly or exclusively xml-related tooling and
support for editing the manual. since docbook is now an implementation
detail (except for still being allowed in option docs, for now) these
tools are no longer necessary, useful, or even functional.
xmlformat.conf is still used by the nixpkgs documentation, so we have to
keep it. there's no reason it can't go live with the nixpkgs docs though.
Building the nixpkgs manual currently triggers a bunch of deprecation
warnings, because every attribute in `lib` is evaluated to see if it's
an attrset to generate locations for.
Instead, share the lib subsets to include in the documentation
between `lib-function-docs` and `lib-function-locations` so they can
coordinate.
Also generate the list of sections instead of duplicating it in
`library.xml`.
