Supporting builddir in cabal.project and global config

To the best of my knowledge, `new-X` don't currently obey `builddir` entries in cabal.project. There was some speculation that there was a technical reason for this, but at first glance of the code it just seems the problem is laziness. That is, all code paths dealing with computing the DistDirLayout try and resolve the cabal.project path and builddir in a single step.

It seems fairly straightforward* to split this into two steps, first looking up cabal.project and the project path, then checking establishing the DistDirLayout after that. I would assume that the logical priority for determining builddir would be:
1) commandline `--builddir` flag
2) builddir in cabal.project
3) default to `dist-newstyle`

If no one has any theoretical objections I'll try and see if I can get it working as straightforwardly as I hope it to be :p

[*] - Famous last words.