Git buildouts now seen from the outside

Instead of using relative paths from the build, which can get hard to
fix if the build dir is itself a broken symlink, such as created precisely with
the subdir option of git/bzr buildout downloaders, manipulate them from the
outside
Made cache directories absolute

Referring to them from the inside of build/ could not work uniformly due
to potential dereferencing of symlinks.
Two prop syntax error introduced in [eba21e4]