audit: check for Python-wide site-package usage

See Homebrew#16662

We would like to enforce vendoring for Python libraries,
or the usage of a virtualenv in the formula's libexec directory,
using a virtualenv.

Library/Homebrew/formula_cellar_checks.rb

@@ -143,6 +143,18 @@ def check_easy_install_pth(lib)
  EOS
  end
 
+ def check_global_site_package_usage(lib)
+ install_found = Dir["#{lib}/python{3}*/site-packages/"].map { |f| File.dirname(f) }
+ return if install_found.empty?
+
+ <<~EOS
+ Python-wide site-packages usage detected. This is not allowed in Homebrew (see PEP 668).
+ Please either vendor this Python library or install it in libexec using a virtualenv.
+ The offending files are:
+ #{install_found * "site-packages/\n "}
+ EOS
+ end
+
  def check_elisp_dirname(share, name)
  return unless (share/"emacs/site-lisp").directory?
  # Emacs itself can do what it wants
@@ -397,6 +409,7 @@ def audit_installed
  problem_if_output(check_non_executables(formula.sbin))
  problem_if_output(check_generic_executables(formula.sbin))
  problem_if_output(check_easy_install_pth(formula.lib))
+ problem_if_output(check_global_site_package_usage(formula.lib))
  problem_if_output(check_elisp_dirname(formula.share, formula.name))
  problem_if_output(check_elisp_root(formula.share, formula.name))
  problem_if_output(check_python_packages(formula.lib, formula.deps))