please test: cabal install now less eager to reinstall core packages

[Duncan Coutts]

Claus, Simon,

You'll recall the cabal-install problem you noticed recently where it
decides it wants to re-install some core library like array or
containers or whatever.

I'd appreciate it if you could test the darcs version of cabal-install
and see if it is behaving better now.


It was choosing to re-install packages when the way it had chosen to
configure the available version of the package (ie what exact
dependencies it'd picked) did not exactly match those of the installed
package.

I've changed things so that it now picks the installed version when the
additional constraints it adds (direct and indirect dependencies) do not
conflict with the existing constraints. This means it'll pick the
installed instance in many more cases and when it does not that there is
a more legitimate reason.

It still may have been possible to use the installed version if we had
known that information much earlier and made different choices at an
earlier stage. I've not yet added support for user supplied install
constraints but I plan to shortly (eg --constraint='array installed').

Duncan