UDITHA JAYASINGHE, ALASDAIR PAL and CHEN LIN SRI Lankan president Gotabaya Rajapaksa has submitted a letter of resignation, two government sources said, after he fled to Singapore following mass protests over his country's economic meltdown. Rajapaksa emailed a letter of resignation to the speaker of the country's parliament late on Thursday, two sources said. It was not immediately clear if the letter, sent shortly after Rajapaksa arrived in Singapore, would be accepted in email form, the sources added. In commercial capital Colombo, troops patrolled the streets to enforce a curfew. Rajapaksa, who fled to the Maldives on Wednesday to escape…