Suppress "not on PATH" warning when --prefix is given

[nchepanov]

What's the problem this feature will solve?

pip install pkg --prefix foo/ triggers WARNING: The scripts ... are installed in 'foo/bin' which is not on PATH.

For example:

➜  python -m pip --version
pip 21.0.1 from .../tmp/.venv/lib/python3.9/site-packages/pip (python 3.9)
➜  python3.9 -m pip install black --prefix foo
...
Successfully built black
Installing collected packages: typed-ast, regex, pathspec, appdirs, black
  WARNING: The scripts black, black-primer and blackd are installed in 'foo/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.
Successfully installed appdirs-1.4.4 black-20.8b1 pathspec-0.8.1 regex-2021.4.4 typed-ast-1.4.3

Describe the solution you'd like

Similar to what's done for --target calls:

pip/src/pip/_internal/commands/install.py

Lines 384 to 385 in 7ec0fa5

	# Don't warn about script install locations if
	# --target has been specified

avoid printing the warning, since it's clear for the context that it's very likely the executables are not going to be on $PATH

Alternative Solutions

It's possible to work around this warning by providing --no-warn-script-location.

Extra context

Interesting, this warning is triggered when using console_scripts, but not when using scripts.

[uranusjr]

Sounds reasonable to me, I’ll be happy to accept a PR!