ghc-pkg should splice $topdir on Windows

On Windows, filepaths in the package database use $topdir rather than including the absolute filesystem location, because the installation is independent of its final resting place. So we get things like this:

> ghc-pkg field base haddock-html     
haddock-html: $topdir\html\libraries\base

Unfortunately this means that Haddock's --use-package flag doesn't work, because it can't figure out what $topdir is. The solution would be to either

• have ghc-pkg splice in the actual value of $topdir, or

• do the Windows installation in the same way as the Unix installation,

  and replace $topdir in package.conf at install time.

The second option probably involves more work.

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture	Multiple