message_about_scripts_not_on_PATH does not expanduser()

Environment

• pip version: 18.1
• Python version: 3.7
• OS: Fedora 29

from python3-pip-18.1-1.fc29.noarch

Description I was trying to install PyGitUp with pip3 install --user git-up and ran into an issue where I was warned:

  The script git-up is installed in '/home/jflorian/.local/bin' which is not on PATH.
  Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location. 

I callled BS because I knew this was on my PATH. But then I noticed my PATH actually had ~/.local/bin and not /home/jflorian/.local/bin as claimed. I looked at wheel.py where the error originates and sure enough, there was no usage of os.path.expanduser(). I changed my PATH to have this already expanded and the error message disappeared.

Expected behavior Most tools seem to know how to expand the tilde in my PATH, I would expect Pip to do so as well.

How to Reproduce

1. export PATH with a local bin directory using a tilde, e.g., ~/.local/bin
2. Install any package with scripts using the --user option.
3. Observe the technically correct warning, but which is misleading w/o serious thought because it must be taken very literally and not as most users are likely to understand.
4. Repeat with PATH having the tilde already expanded, e.g., /home/jflorian/.local/bin and observe the behavior I’d expect in step 3.

[chrahunt]

What if we warn about this case if we see a path in PATH that starts with ~? For example:

The script git-up is installed in '/home/jflorian/.local/bin' which is not on PATH.
  Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
NOTE: The current PATH contains path(s) starting with `~`, which may not be expanded by all applications.

[chrahunt]

Closed in #7345. Thanks @mdebi!