Odd paths on Windows

[igloo]

If, on Windows, I do

ghc --make Setup.hs
./Setup configure --prefix=c:/cygwin/home/ian/local/haskell
./Setup haddock
./Setup build
./Setup install --user

then cabal puts the binaries under c:/cygwin/home/ian/local/haskell/Haskell/, which is a bit odd. However, it puts the documentation in c:/Program Files/Common Files/mtl-1.0/, which is even odder. If we are going to put things here then a Haskell/ directory makes a lot of sense, but better still would be to put the docs somewhere under the prefix that the user asked us to install under.

[duncan]

We should fix this.

It's easy to move "\Haskell" into the $prefix rather than where it is currently in the $bindir and $libdir defaults.

It's now really easy to change it's all specified clearly in Distribution.Simple.InstallDirs?.defaultInstallDirs, we just need to decide what defaults we actually want on Windows.

[duncan]

The prefix on Windows now includes the trailing Haskell, so if you override prefix then you don't get any Haskell sub dir. Also, the docs are now installed relative to prefix. And the user guide has been updated.