[darwinbuild] Moved .xref.db into .build

[Kevin Van Vechten]

For those of you working off top-of-tree darwinbuild, I've just moved  
the xref.db inside the .build directory.  Sorry for the seemingly  
gratuitous change, you should update any of you current darwinbuild  
working directories with:

$ mv .xref.db .build/xref.db

If anyone has any other suggestions regarding the .build directory,  
please say so now, since I'd like to make its format mostly finalized  
before the 0.6 release.

Thanks!

- Kevin