pth files are not processed on Ubuntu under Python 2 pypa#1517

If the host platfrom and pure library paths differed from the
distutils installs ones, we patched the sys.path by appending the
missing paths, this did not trigger the pth processing though.
Changed to use the site.add_site_dir to solve the problem.

Signed-off-by: Bernat Gabor <bgabor8@bloomberg.net>

docs/changelog/1517.bugfix.rst

@@ -0,0 +1 @@
+pth files were not processed under Debian CPython2 interpreters - by ``gaborbernat``.

src/virtualenv/create/via_global_ref/builtin/python2/site.py

@@ -22,22 +22,36 @@ def main():
 
 def load_host_site():
     """trigger reload of site.py - now it will use the standard library instance that will take care of init"""
-    # the standard library will be the first element starting with the real prefix, not zip, must be present
-    custom_site_package_path = __file__
+    # we have a duality here, we generate the platform and pure library path based on what distutils.install specifies
+    # because this is what pip will be using; the host site.py though may contain it's own pattern for where the
+    # platform and pure library paths should exist
+
+    # notably on Ubuntu there's a patch for getsitepackages to point to
+    # - prefix + local/lib/pythonx.y/dist-packages
+    # - prefix + lib/pythonx.y/dist-packages
+    # while distutils.install.cmd still points both of these to
+    # - prefix + lib/python2.7/site-packages
+
+    # to facilitate when the two match, or not we first reload the site.py, now triggering the import of host site.py,
+    # as this will ensure that initialization code within host site.py runs
+
+    here = __file__  # the distutils.install patterns will be injected relative to this site.py, save it here
     reload(sys.modules["site"])  # noqa
 
-    # ensure that our expected site packages is on the sys.path
+    # and then if the distutils site packages are not on the sys.path we add them via add_site_dir; note we must add
+    # them by invoking add_site_dir to trigger the processing of pth files
     import os
 
     site_packages = r"""
     ___EXPECTED_SITE_PACKAGES___
     """
     import json
 
+    add_site_dir = sys.modules["site"].addsitedir
     for path in json.loads(site_packages):
-        full_path = os.path.abspath(os.path.join(custom_site_package_path, path.encode("utf-8")))
+        full_path = os.path.abspath(os.path.join(here, path.encode("utf-8")))
         if full_path not in sys.path:
-            sys.path.append(full_path)
+            add_site_dir(full_path)
 
 
 def read_pyvenv():