Remove sys.path entry for vendored pydevd after loading it.

Improve error reporting when pydevd is imported before ptvsd.

ptvsd/__init__.py

@@ -5,10 +5,32 @@
 __author__ = "Microsoft Corporation <ptvshelp@microsoft.com>"
 __version__ = "4.0.0a1"
 
-# ptvsd must always be imported before pydevd
 import sys
-assert 'pydevd' not in sys.modules
+import os.path
+
+# ptvsd must always be imported before pydevd
+if 'pydevd' in sys.modules:
+    raise ImportError('ptvsd must be imported before pydevd')
 
 # Add our vendored pydevd directory to path, so that it gets found first.
-import os.path
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), 'pydevd'))
+
+# Load our wrapper module, which will detour various functionality inside pydevd.
+# This must be done before the imports below, otherwise some modules will end up
+# with local copies of pre-detour functions.
+import ptvsd.wrapper
+
+# Now make sure all the top-level modules and packages in pydevd are loaded.
+import _pydev_bundle
+import _pydev_imps
+import _pydev_runfiles
+import _pydevd_bundle
+import _pydevd_frame_eval
+import pydev_ipython
+import pydevd_concurrency_analyser
+import pydevd_plugins
+import pydevd
+
+# Remove sys.path entry added above - any pydevd modules that aren't loaded at
+# this point, will be loaded using their parent package's __path__.
+del sys.path[0]

ptvsd/wrapper.py

@@ -2,8 +2,7 @@
 # Licensed under the MIT License. See LICENSE in the project root
 # for license information.
 
-# TODO: with_statement is not needed
-from __future__ import print_function, with_statement, absolute_import
+from __future__ import print_function, absolute_import
 
 import atexit
 import os