Check if requirements.txt exist before proceeding further inspection

[pkesavap]

Feel free to close this issue, if already implemented.
I ran pipreqs ./ to generate requirements.txt without knowing i already had one, and after about 4 minutes, pipreqs shows the warning

WARNING: requirements.txt already exists, use --force to overwrite it

Maybe we could do that check first before proceeding with other introspection?

Thanks

[pkesavap]

Version:
pipreqs --version
0.4.11

[clvnkhr]

Hi, I'm new to this but the fix was simple so I went ahead and opened a pull request. Tox passed locally but I think I need someone else in order to proceed. Please tell me if I did something wrong or if theres anything else I should do :)