Fix various setup.py processing bugs

[ghost]

I made the setup.py processing unintentionally litter a file 😬 😬 (in the user project folder of all places) so that's not very good and here's the fix 😄 along with other fixes:

• fix file litter of constraint.txt
• fix --private relative paths during build.py's setup.py install
• fix "" and "." not being recognized as folder by pythonpackage.py

Still WIP because I'm still testing it tested with various actual setup.py-based builds!

[ghost]

For what it's worth, the coverage thing: there is no new code except 2 conditionals that were plain missing, is this computed based on functions present? I did split up a big one (as once suggested by @AndreMiras !) simply because it got too nested to read well, but there isn't really any new code - and I actually added tests for the broken handling of pythonpackage of "" and ".", so in a way there's more tested than before 🤷‍♀️

Edit: I'm not objecting to having it tracked though, just slightly surprised at the way it's computed 😄

[AndreMiras]

Thanks I indeed like that you could split the code in a meaningful way with that run_setuppy_install() method. I feel like it makes things a bit more readable and surely more testable.
As for the coverage, well it only complains on a decrease by 0.02% which is nothing. It's probably simply because you added a couple of lines in a file that already had low coverage. Of course I hope nobody is going to block a PR because the coverage decreased by such a tiny amount 😄