The commerce power is not confined in its exercise to the regulation of commerce among the states. It extends to those activities intrastate which so affect interstate commerce, or the exertion of the power of Congress over it, as to make regulation of them appropriate means to the attainment of a legitimate end, the effective execution of the granted power to regulate interstate commerce... The power of Congress over interstate commerce is plenary and complete in itself, may be exercised to its utmost extent, and acknowledges no limitations other than are prescribed in the Constitution... It follows that no form of state activity can constitutionally thwart the regulatory power granted by the commerce clause to Congress. Hence, the reach of that power extends to those intrastate activities which in a substantial way interfere with or obstruct the exercise of the granted power.