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 +143,25 @@ def check_easy_install_pth(lib)
     EOS
   end
 
+  def check_global_site_package_usage(formula)
+    return if !formula.tap.core_tap?
+    return if !formula.stable.url.match?("https://files.pythonhosted.org")
+
+    lib = formula.lib
+    global_site_package_found = Dir["#{lib}/python{3}*/site-packages/"].map { |f| File.dirname(f) }
+    return if global_site_package_found.empty?
+
+    bindings_found = Dir["#{lib}/python{3}*/site-packages/**/*.so"].map { |f| File.dirname(f) }
+    return if !bindings_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:
+        #{global_site_package_found * "\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 +416,7 @@ 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))
     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))