PyPDF2 is missing from output with --use-local (although installed)

`PyPDF2` is installed and I import it in my project, but when I use `pipreqs --use-local`, it is missing from the output, while it is included in the output of `pipreqs`:

```
user@host:~/temp> pip freeze | grep PyPDF2
PyPDF2==1.26.0

user@host:~/temp> ll
total 4
-rw-r--r-- 1 user group 38 Mar 6 12:46 my.py

user@host:~/temp> cat my.py
import PyPDF2
print(PyPDF2._version)

user@host:~/temp> python my.py

user@host:~/temp> pipreqs --print
PyPDF2==1.26.0
INFO: Successfully output requirements

user@host:~/temp> pipreqs --print --use-local

INFO: Successfully output requirements
```