Without this patch `virtualenv --python=python2.7 .` fails with an
error because it notices that the python readline.so is not in the
same path as python2.7. I assume this is to avoid copying the wrong
file on systems where it is possible to find incompatible libraries by
accident. Adding "/nix/store" to the prefix fixes this problem.

A sitecustomize.py is created in the virtualenv which makes libraries
from the python specified by the --python argument available to the
virtualenv. For example, this makes readline and sqlite3 available
when a wrapped python is specified. If no --python argument is passed,
it will only add the path to the python used when building
`virtualenv`, which is the unwrapped python, so sqlite3 won't be
available.


diff --git a/virtualenv.py b/virtualenv.py
index bcf3225..3530997 100755
--- a/virtualenv.py
+++ b/virtualenv.py
@@ -1163,20 +1163,7 @@ def path_locations(home_dir, dry_run=False):
 
 
 def change_prefix(filename, dst_prefix):
-    prefixes = [sys.prefix]
-
-    if IS_DARWIN:
-        prefixes.extend(
-            (
-                os.path.join("/Library/Python", sys.version[:3], "site-packages"),
-                os.path.join(sys.prefix, "Extras", "lib", "python"),
-                os.path.join("~", "Library", "Python", sys.version[:3], "site-packages"),
-                # Python 2.6 no-frameworks
-                os.path.join("~", ".local", "lib", "python", sys.version[:3], "site-packages"),
-                # System Python 2.7 on OSX Mountain Lion
-                os.path.join("~", "Library", "Python", sys.version[:3], "lib", "python", "site-packages"),
-            )
-        )
+    prefixes = ["/nix/store", sys.prefix]
 
     if hasattr(sys, "real_prefix"):
         prefixes.append(sys.real_prefix)
@@ -1199,6 +1186,8 @@ def change_prefix(filename, dst_prefix):
             if src_prefix != os.sep:  # sys.prefix == "/"
                 assert relative_path[0] == os.sep
                 relative_path = relative_path[1:]
+                if src_prefix == "/nix/store":
+                    relative_path = "/".join(relative_path.split("/")[1:])
             return join(dst_prefix, relative_path)
     assert False, "Filename {} does not start with any of these prefixes: {}".format(filename, prefixes)
 
@@ -1375,6 +1364,11 @@ def install_python(home_dir, lib_dir, inc_dir, bin_dir, site_packages, clear, sy
     site_filename_dst = change_prefix(site_filename, home_dir)
     site_dir = os.path.dirname(site_filename_dst)
     writefile(site_filename_dst, SITE_PY)
+    wrapper_path = join(prefix, "lib", py_version, "site-packages")
+    writefile(
+        join(site_dir, 'sitecustomize.py',),
+        "import sys; sys.path.append('%s')" % wrapper_path
+    )
     writefile(join(site_dir, "orig-prefix.txt"), prefix)
     site_packages_filename = join(site_dir, "no-global-site-packages.txt")
     if not site_packages: