Haddock version during build

[Claus Reinke]

>> 1. register other paths that may change with (re-)installation
>>    with ghc-pkg, ..
> Yes, that could be done.  It's a bit yucky though.  Maybe we should have an 
> XML configuration file like everybody else.

I'm not a great fan of XML, but no matter the format, the info
needs to be stored in an accessible way. package.conf seems
to be the only currently available option.
 
> Remember that storing libdir in package.conf is strictly superfluous - you 
> already need to know libdir in order to find package.conf in the first place.

Superfluos in principle, but it would be nice to have all paths
in a single place for consistency. And then there's practice: 
in a GHCi session, the session itself finds the right package.conf
(it is just GHC --interactive, after all), but how do I get access 
to that information from the GHCi prompt (I often find myself
wondering which packages are available for the particular GHCi
in use, #2345)? In GHC API code, the compiling GHC finds
package.conf, but how can that information be used in the 
compiled code (see this thread, for instance;-)?

Claus