The ornamental design for a portable lightsource, as shown and described.
FIG. 1 is a front elevational view of a portable light source according to the present invention.
FIG. 2 is a rear elevational view of the portable light source.
FIG. 3 is a top plan view of the portable light source.
FIG. 4 is a bottom plan view of the portable light source; and,
FIG. 5 is a right side elevational view of the portable light source, with the left side elevational view being a mirror image of the left side elevational view.