Thirty years ago, the founder of Philharmonia Baroque Orchestra, early music crusader and harpsichordist Laurette Goldberg tapped Nicholas McGegan to become Philharmonia’s music director. Under “Nic’s” leadership, a five-year-old ensemble previously run by four ensemble musicians who conducted orchestra business in a spare bedroom in the back of soprano Judith Nelson’s house was transformed into one of the world’s most captivating period ensembles.