Hack around longstanding coverage gotcha.

We were always having our 1st sys.path entry nuked, but it just so
happened the 0th position used to be held by the pex bootstrap which was
no longer needed by the time coverage ran.

src/python/pants/backend/python/tasks/pytest_prep.py

@@ -8,6 +8,9 @@
 from builtins import object
 
 import pkg_resources
+from pex.interpreter import PythonInterpreter
+from pex.pex import PEX
+from pex.pex_builder import PEXBuilder
 from pex.pex_info import PexInfo
 
 from pants.backend.python.subsystems.pytest import PyTest
@@ -23,8 +26,15 @@ class PytestBinary(object):
 
     _COVERAGE_PLUGIN_MODULE_NAME = '__{}__'.format(__name__.replace('.', '_'))
 
-    def __init__(self, pex):
-      self._pex = pex
+    def __init__(self, interpreter, pex):
+      # Here we hack around `coverage.cmdline` nuking the 0th element of `sys.path` (our root pex)
+      # by ensuring, the root pex is on the sys.path twice.
+      # See: https://github.com/nedbat/coveragepy/issues/715
+      pex_path = pex.path()
+      pex_info = PexInfo.from_pex(pex_path)
+      pex_info.merge_pex_path(pex_path)  # We're now on the sys.path twice.
+      PEXBuilder(pex_path, interpreter=interpreter, pex_info=pex_info).freeze()
+      self._pex = PEX(pex=pex_path, interpreter=interpreter)
 
     @property
     def pex(self):
@@ -76,4 +86,6 @@ def execute(self):
     pex_info = PexInfo.default()
     pex_info.entry_point = 'pytest'
     pytest_binary = self.create_pex(pex_info)
-    self.context.products.register_data(self.PytestBinary, self.PytestBinary(pytest_binary))
+    interpreter = self.context.products.get_data(PythonInterpreter)
+    self.context.products.register_data(self.PytestBinary,
+                                        self.PytestBinary(interpreter, pytest_binary))