Revert "Revert "Revert "Add path interposer"""

This reverts commit c59a92cb1a.

Reason for revert: tests are broken with long OUT_DIRs
They're directly calling net.Listen, and not using the fallback
for long socket names.

Change-Id: Id14cbd499fd9b36c6926b7552d3554340cb0916c

python/scripts/stub_template_host.txt

@@ -18,6 +18,8 @@ def SearchPathEnv(name):
   for directory in search_path:
     if directory == '': continue
     path = os.path.join(directory, name)
+    if os.path.islink(path):
+      path = os.path.realpath(path)
     # Check if path is actual executable file.
     if os.path.isfile(path) and os.access(path, os.X_OK):
       return path