The ornamental design for a rainwatercollector, as shown and described.
FIG. 1 is an isometric view of the rainwater collector;
FIG. 2 is an elevation view of a first side thereof;
FIG. 3 is a front view thereof;
FIG. 4 is an elevation view of a second side thereof; and,
FIG. 5 is a top view thereof.
Views of the bottom side and the back side of the rainwater collector have been omitted because such sides are flat and devoid of surface ornamentation.