feat: add new build_dir field in settings

[shubhbapna]

fixes #278

had to tell mypy to ignore settings.py until #257

[shubhbapna]

@dhellmann now that i think about this, should i just apply this transformation: ctx.settings.build_dir(req.name, sdist_root_dir) in prepare_source and find_source_dir. What do you think?

[shubhbapna]

Handling the case where user tries to escape the source root dir is a bit tough since ".." can appear anywhere in the path for example: foo/../../../bar will allow me to escape outside the source root dir. So I used a hack - if ".." appears anywhere in the path then resolve it to an absolute path and since I have code that can handle absolute path we are able to contain the directory escaping.

For example say our sdist dir path is /home/outside/sdist_root_dir and build_dir is defined as ../outside then settings.build_dir will return /home/outside/sdist_root_dir/home/outside because ../outside is resolved to /home/outside.