python: do not add main brewed Python site-package to virtualenv

If added, this makes the virtualenv read the main site-package from brewed Python,
and especially makes it read our sitecustomize.py file, which will
modify the sys.executable path.

See the full discussion at:
#8873

I also took the opportunity to not include test deps, as these will
be not be installed, so the .pth file should not contains references
to site-packages from test deps.

Previous packages on Linux did already contain the wrong lines in the pth file,
for example:
cat /home/linuxbrew/.linuxbrew/Cellar/aws-google-auth/0.0.36_1/libexec/lib/python3.8/site-packages/homebrew_deps.pth
import site; site.addsitedir('/home/linuxbrew/.linuxbrew/opt/[email protected]/lib/python3.8/site-packages')
import site; site.addsitedir('/home/linuxbrew/.linuxbrew/opt/libxml2/lib/python3.8/site-packages')

This might have caused subtle bugs for some packages but not for others.

Library/Homebrew/language/python.rb

@@ -159,7 +159,10 @@ def virtualenv_create(venv_root, python = "python", formula = self)
         # Find any Python bindings provided by recursive dependencies
         formula_deps = formula.recursive_dependencies
         pth_contents = formula_deps.map do |d|
-          next if d.build?
+          next if d.build? || d.test?
+          # Do not add the main site-package provided by the brewed
+          # Python formula, to keep the virtual-env's site-package pristine
+          next if python_names.include? d.name
 
           dep_site_packages = Formula[d.name].opt_prefix/Language::Python.site_packages(python)
           next unless dep_site_packages.exist?
@@ -196,8 +199,7 @@ def needs_python?(python)
       def virtualenv_install_with_resources(options = {})
         python = options[:using]
         if python.nil?
-          pythons = %w[python python3 pypy pypy3] + Formula.names.select { |name| name.start_with? "python@" }
-          wanted = pythons.select { |py| needs_python?(py) }
+          wanted = python_names.select { |py| needs_python?(py) }
           raise FormulaUnknownPythonError, self if wanted.empty?
           raise FormulaAmbiguousPythonError, self if wanted.size > 1
 
@@ -210,6 +212,10 @@ def virtualenv_install_with_resources(options = {})
         venv
       end
 
+      def python_names
+        %w[python python3 pypy pypy3] + Formula.names.select { |name| name.start_with? "python@" }
+      end
+
       # Convenience wrapper for creating and installing packages into Python
       # virtualenvs.
       class Virtualenv