make py lib a self-contained directory again

- move and merge _py/ bits back to py/
- fixes all around

--HG--
branch : trunk

testing/plugin/test_pytest_tmpdir.py

@@ -0,0 +1,9 @@
+from py.plugin.pytest_tmpdir import pytest_funcarg__tmpdir
+
+def test_funcarg(testdir):
+    from py.impl.test.funcargs import FuncargRequest
+    item = testdir.getitem("def test_func(tmpdir): pass")
+    p = pytest_funcarg__tmpdir(FuncargRequest(item))
+    assert p.check()
+    bn = p.basename.strip("0123456789-")
+    assert bn.endswith("test_func")