Concurrent imports cause error in force_pydevd

[int19h]

Happened in Flask multiproc test on macOS, but I think it can potentially occur in any scenario where a module is imported concurrently from another thread:

I+05.947: Debuggee-86 stderr:
          b'Error on request:\n'
          b'Traceback (most recent call last):\n'
          b'  File "/Users/runner/runners/2.159.2/work/1/s/.tox/py37/lib/python3.7/site-packages/werkzeug/serving.py", line 304, in run_wsgi\n'
          b'    execute(self.server.app)\n'
          b'  File "/Users/runner/runners/2.159.2/work/1/s/.tox/py37/lib/python3.7/site-packages/werkzeug/serving.py", line 292, in execute\n'
          b'    application_iter = app(environ, start_response)\n'
          b'  File "/Users/runner/runners/2.159.2/work/1/s/.tox/py37/lib/python3.7/site-packages/flask/_compat.py", line 36, in reraise\n'
          b'    raise value\n'
          b'  File "/Users/runner/runners/2.159.2/work/1/s/tests/test_data/flask1/__init__.py", line 1, in <module>\n'
          b'    import debug_me  # noqa\n'
          b'  File "/Users/runner/runners/2.159.2/work/1/s/tests/DEBUGGEE_PYTHONPATH/debug_me/__init__.py", line 49, in <module>\n'
          b'    import ptvsd.server\n'
          b'  File "/Users/runner/runners/2.159.2/work/1/s/.tox/py37/lib/python3.7/site-packages/ptvsd/server/__init__.py", line 9, in <module>\n'
          b'    import ptvsd._vendored.force_pydevd  # noqa\n'
          b'  File "/Users/runner/runners/2.159.2/work/1/s/.tox/py37/lib/python3.7/site-packages/ptvsd/_vendored/force_pydevd.py", line 14, in <module>\n'
          b"    prefix_matcher('pydev', '_pydev'))\n"
          b'  File "/Users/runner/runners/2.159.2/work/1/s/.tox/py37/lib/python3.7/site-packages/ptvsd/_vendored/__init__.py", line 101, in check_modules\n'
          b'    for modname, mod in sys.modules.items():\n'
          b'RuntimeError: dictionary changed size during iteration\n'

It needs to take a snapshot of the module list before iterating.