avoid undocumented special casing of "pytest_" prefix

--HG--
branch : plugin_no_pytest

_pytest/config.py

@@ -208,8 +208,6 @@ class PytestPluginManager(PluginManager):
         try:
             mod = importplugin(modname)
         except ImportError:
-            if modname.startswith("pytest_"):
-                return self.import_plugin(modname[7:])
             raise
         except Exception as e:
             import pytest

testing/test_pytester.py

@@ -64,7 +64,7 @@ def test_parseconfig(testdir):
 
 def test_testdir_runs_with_plugin(testdir):
     testdir.makepyfile("""
-        pytest_plugins = "pytest_pytester"
+        pytest_plugins = "pytester"
         def test_hello(testdir):
             assert 1
     """)

testing/test_recwarn.py

@@ -22,7 +22,6 @@ def test_WarningRecorder(recwarn):
 
 def test_recwarn_functional(testdir):
     reprec = testdir.inline_runsource("""
-        pytest_plugins = 'pytest_recwarn',
         import warnings
         oldwarn = warnings.showwarning
         def test_method(recwarn):

testing/test_unittest.py

@@ -3,7 +3,6 @@ import pytest
 def test_simple_unittest(testdir):
     testpath = testdir.makepyfile("""
         import unittest
-        pytest_plugins = "pytest_unittest"
         class MyTestCase(unittest.TestCase):
             def testpassing(self):
                 self.assertEquals('foo', 'foo')
@@ -17,7 +16,6 @@ def test_simple_unittest(testdir):
 def test_runTest_method(testdir):
     testdir.makepyfile("""
         import unittest
-        pytest_plugins = "pytest_unittest"
         class MyTestCaseWithRunTest(unittest.TestCase):
             def runTest(self):
                 self.assertEquals('foo', 'foo')