WASHINGTON — Drugmaker Purdue Pharma, the company behind OxyContin, the powerful prescription painkiller that experts say helped touch off an opioid epidemic, will plead guilty to federal criminal charges as part of a settlement of more than $8 billion, the U.S. Justice Department announced Wednesday. The deal does not release any of the company’s executives or owners — members of the wealthy Sackler family — from criminal liability, and a criminal investigation is ongoing.