Phase Stability of Lead Phosphate Apatite Pb<sub>10–x</sub>Cu<sub>x</sub>(PO<sub>4</sub>)<sub>6</sub>O, Pb<sub>10–x</sub>Cu<sub>x</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub> (x = 0, 1), and Pb<sub>8</sub>Cu<sub>2</sub>(PO<sub>4</sub>)<sub>6</sub>