Allow disabling of the sys.path rewrite technique using an environment variable.

CHANGES.txt

@@ -2,6 +2,21 @@
 CHANGES
 =======
 
+----
+18.3
+----
+
+* Setuptools now allows disabling of the manipulation of the sys.path
+  during the processing of the easy-install.pth file. To do so, set
+  the environment variable ``SETUPTOOLS_SYS_PATH_TECHNIQUE`` to
+  anything but "rewrite" (consider "raw"). During any install operation
+  with manipulation disabled, setuptools packages will be appended to
+  sys.path naturally.
+
+  Future versions may change the default behavior to disable
+  manipulation. If so, the default behavior can be retained by setting
+  the variable to "rewrite".
+
 ----
 18.2
 ----

setuptools/command/easy_install.py

@@ -1612,7 +1612,8 @@ class RewritePthDistributions(PthDistributions):
         """)
 
 
-PthDistributions = RewritePthDistributions
+if os.environ.get('SETUPTOOLS_SYS_PATH_TECHNIQUE', 'rewrite') == 'rewrite':
+    PthDistributions = RewritePthDistributions
 
 
 def _first_line_re():