Module not found error with rospkg when trying to import roslib

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Hello,

I have developed a custom control algorithm for mz robot in gazebo. Untile yesterday I used to use

roslaunch spider_gazebo spider_world.launch

to spawn the robot and load control configurations in gazebo and then in another terminal i used to export ROS_MASTER_URI and then use

rosrun spider_control control.py

to run my custom python plugin. Until yesterday this was working well, but today when I tried to launch again `I am having the following error

    'Traceback (most recent call last):
   File "/home/microbot/catkin_ws/src/spider/spider_control/control.py", line 5, in <module>
     import roslib
   File "/opt/ros/melodic/lib/python2.7/dist-packages/roslib/__init__.py", line 50, in <module>
     from roslib.launcher import load_manifest
    File "/opt/ros/melodic/lib/python2.7/dist-packages/roslib/launcher.py", line 42, in <module>
      import rospkg
    ModuleNotFoundError: No module named 'rospkg'

I have also tried the following command to check whether my python path is right or not ,

 'echo $PYTHONPATH'

The output is

  '/home/microbot/catkin_ws/devel/lib/python2.7/dist-packages:/opt/ros/melodic/lib/python2.7/dist- 
    packages:/opt/ros/melodic/share/ros/core/roslib/src

I can not understand whats causing this error. can any one please help me point out the reason. Thanks