Make pipreqs not write duplicate lines to requirements.txt file

[dimp-gh]

Test file:

import pymongo
from bson.objectid import ObjectId

For this file pipreqs generates the following requirements.txt:

pymongo==3.2.1
pymongo==3.2.1

This pull-request fixes this unfortunate behaviour by patching get_pkg_names function.

[bndr]

Thanks! Looks good