Varying input voltage, constant current and varying output voltage from a buck converter?
Basically what you're asking is, do current controlled switching convertes exist.
The answer is yes, as you can see here:



Yes, that looks complicated ! But it's not (for me at least).
The MAX649 is a DCDC Buck converter controller IC, it does not have a build in switch,  PMOS Q1 is the switch. If you would want a constant output VOLTAGE, you would need a voltage divider (2 resistors) and feed a divided value of the output voltage back to the FB (feedback) pin.

But in this circuit, has a constant output CURRENT, to charge a battery ! Instead of sensing the output voltage, now the output current is sensed by taking the voltage across R10, amplifying this voltage (the opamp does this, with a feedback network to set the gain) and where does ths feedback signal go ? Straight back in to the FB pin !

The simple buck converter in your schematic is too simple to be considered having a voltage or a current output. If you change the load then voltage and current change, it is not regulated in any way. Yes you can change the PWM of the switch signal but that PWM signal needs te be determined somehow. This is done with voltage or current feedback.