feat(pip): license support

[knqyf263]

Description

Trivy currently doesn't support licenses for requirements.txt.

trivy/docs/docs/coverage/language/python.md

Line 8 in 9d26ae8

| pip | ✓ | ✓ | - |

We probably need to guess the path of site-packages and look into the directory for licenses. The python code is like this:

   for path in sys.path:
        if 'site-packages' in path:
            return path

    if hasattr(sys, 'real_prefix') or (hasattr(sys, 'base_prefix') and sys.base_prefix != sys.prefix):
        return os.path.join(sys.prefix, 'lib', 'python{}.{}'.format(*sys.version_info), 'site-packages')