Fix the vendoring script to cover relative imports

src/pip/_vendor/pkg_resources/py31compat.py

@@ -2,7 +2,7 @@ import os
 import errno
 import sys
 
-from .extern import six
+from pip._vendor import six
 
 
 def _makedirs_31(path, exist_ok=False):

src/pip/_vendor/pyparsing.py

@@ -37,7 +37,7 @@ C{"<salutation>, <addressee>!"}), built up using L{Word}, L{Literal}, and L{And}
 (L{'+'<ParserElement.__add__>} operator gives L{And} expressions, strings are auto-converted to
 L{Literal} expressions)::
 
-    from pyparsing import Word, alphas
+    from pip._vendor.pyparsing import Word, alphas
 
     # define grammar of a greeting
     greet = Word(alphas) + "," + Word(alphas) + "!"
@@ -1594,7 +1594,7 @@ class ParserElement(object):
            after importing pyparsing.
            
            Example::
-               import pyparsing
+               from pip._vendor import pyparsing
                pyparsing.ParserElement.enablePackrat()
         """
         if not ParserElement._packratEnabled:

tasks/vendoring/__init__.py

@@ -75,7 +75,8 @@ def rewrite_file_imports(item, vendored_libs):
     """Rewrite 'import xxx' and 'from xxx import' for vendored_libs"""
     text = item.read_text(encoding='utf-8')
     # Revendor pkg_resources.extern first
-    text = re.sub(r'pkg_resources.extern', r'pip._vendor', text)
+    text = re.sub(r'pkg_resources\.extern', r'pip._vendor', text)
+    text = re.sub(r'from \.extern', r'from pip._vendor', text)
     for lib in vendored_libs:
         text = re.sub(
             r'(\n\s*|^)import %s(\n\s*)' % lib,