just log warnings when --user installs hide behind virtualenv installs

[qwcode]

currently, users can fail with Will not install to the user site because it will lack sys.path precedence when doing --user installs that will be hidden behind virtualenv site packages (in virtualenvs with system access, the precedence is virtualenv, then user, then global).

I'm considering whether these installs should just warn in this case, and not fail.

the reason to warn (vs fail) is in the scenario, where only one dependency will fail, as part of a larger install of many packages.

[skyl]

I think the optimal solution might be to add a --force or similar that will install to the location regardless of the conflict/precedence

[qwcode]

this is a very special case though. not many people use --user, and even less like this.
not sure I want to add an option for it.

[skyl]

the hack of all hacks that seems to be working for me right now is to have a sh wrapper around pip that seds activate_this.py to remove sys.real_prefix = sys.prefix and then after I run pip, I replace the original activate_this.py.

[skyl]

Well, the above hack no longer works in 1.5.1 and I can't figure out why. I guess sys.real_prefix gets set in a slightly different way... haven't quite figured it out.

[dstufft]

This has been an error for 3+ years at this point and using --user while in a virtual environment is most likely an accident. I'm going to close this as I don't think moving to a warning is going to be helpful here.