Module SCons.Sig

Place-holder for the old SCons.Sig module hierarchy

This is no longer used, but code out there (such as the NSIS module on
the SCons wiki) may try to import SCons.Sig.  If so, we generate a warning
that points them to the line that caused the import, and don't die.

If someone actually tried to use the sub-modules or functions within
the package (for example, SCons.Sig.MD5.signature()), then they'll still
get an AttributeError, but at least they'll know where to start looking.