Does a real "megapixel lens" exist? Many of new model of lenses are defined as megapixel (i.e 1MP, 2MP, 5MP...).

My problem is that I did not find any difference between a standard lens and a "megapixel" one, in the focus quality.
I suppose that a difference should be remarked in the focus quality during the calibration step.

I'm working with several megapixel sensors, and I'm still looking for a megapixel lens that could fit with my sensors.
For example with a 2MP sensor, with a pixel size equal to 4um, the MTF should be at least 0.5 at 125 lp/mm. 
My working distance is about 15m and the illumination wavelength is 850nm.
This is a misnomer, if not pure marketing corruption of terms. The lens doesn't do anything to the "megapixel count", that's determined solely by the sensor.
At most the lens might have the capability to resolve details to a level where a sensor of at least a certain element density is required to record them, but then you'd still not have a number of pixels as the size of the sensor as well as its element density determines that.