Optimized ScorePackageManager initialization.

ScorePackageManager now initializes very quickly:

    def __init__(self, path=None, session=None):
        PackageManager.__init__(
            self,
            path=path,
            session=session,
            )

The way that this is possible is that the 8 or 9 delegate classes aggregated
into ScorePackageManager are now implemented as cached properties.