Package geocode handles mapping user-entered locations into lat/long polygons.
AltLookupFn provides alternative geocode lookup in tests.
If AltLookupFn is not nil, Lookup calls AltLookupFn first, and returns the results unless it is (nil, nil).
Lookup performs its standard lookup using its cache and the Google geocoding serice if AltLookupFn is nil, or it returns (nil, nil) for the address being looked up.
It's up to the caller to change AltLookupFn only when Lookup is not being called.
Lookup returns rectangles for the given address. Currently the only implementation is the Google geocoding service.