Sets the install directories by applying the install schemes.

def select_scheme(self, name):
        """Sets the install directories by applying the install schemes."""
        # it's the caller's problem if they supply a bad name!
        scheme = INSTALL_SCHEMES[name]
        for key in SCHEME_KEYS:
            attrname = 'install_' + key
            if getattr(self, attrname) is None:
                setattr(self, attrname, scheme[key])