Speed up project re-resolving by skipping the loading from file.

When re-resolving an existing project, the build graph already
exists in memory and does not have to be read from disk.

Change-Id: Icf5cf7396a859d4ee1c495b8cc4fa7a1ee61ff1d
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>