use pip via module

Author: Vladimir Kotal

dev/install-python-packages.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
 
-pip3 install --upgrade pip
-pip3 install -r opengrok-tools/src/main/python/requirements.txt
-pip3 install pep8 flake8 virtualenv pylint
+python3 -m pip install --upgrade pip
+python3 -m pip install -r opengrok-tools/src/main/python/requirements.txt
+python3 -m pip install pep8 flake8 virtualenv pylint