PARIS — In the shadow of the famed Sacre Coeur basilica, hundreds of migrants have poured into a makeshift tent camp on a bridge over Eurostar train tracks to Britain — the country that many see as a cherished final destination. Few know who provided the tents; the city set up portable toilets; and municipal crews sweep and power-spray the area regularly. France, like many other rich countries, has faced a dilemma in the burgeoning global flow of migrants fleeing war, poverty or persecution — from Rohingya Muslims drifting in the Straits of Malacca to Africans drowning in the Mediterranean while trying to reach Europe. In a densely populated city, where large unoccupied spaces are rare, the football field-size patch near Paris’ La Chapelle metro station has seen various waves of migration over the years, including Kurds and Afghans. Authorities cite either the flow from a recent surge of crossings to Italy, or a retreat by those expelled by police from Calais, the French city on the English Channel seen by many migrants as a springboard into Britain. [...] after the recent swell, he and other local leaders want to step up the pace and are breaking their silence — hoping to press the government of President Francois Hollande, a fellow Socialist, into finding a solution.