Convert pytest.py into a package

As discussed in https://github.com/pytest-dev/pytest/issues/3342, this
is the first step to make pytest support static typing fully

setup.cfg

@@ -38,8 +38,8 @@ packages =
     _pytest.assertion
     _pytest.config
     _pytest.mark
+    pytest
 
-py_modules = pytest
 python_requires = >=3.5
 
 [options.entry_points]

src/pytest/__init__.py

@@ -4,6 +4,7 @@ pytest: unit and functional testing with Python.
 """
 from _pytest import __version__
 from _pytest.assertion import register_assert_rewrite
+from _pytest.compat import _setup_collect_fakemodule
 from _pytest.config import cmdline
 from _pytest.config import hookimpl
 from _pytest.config import hookspec
@@ -93,14 +94,6 @@ __all__ = [
     "yield_fixture",
 ]
 
-if __name__ == "__main__":
-    # if run as a script or by 'python -m pytest'
-    # we trigger the below "else" condition by the following import
-    import pytest
 
-    raise SystemExit(pytest.main())
-else:
-
-    from _pytest.compat import _setup_collect_fakemodule
-
-    _setup_collect_fakemodule()
+_setup_collect_fakemodule()
+del _setup_collect_fakemodule

src/pytest/__main__.py

@@ -0,0 +1,7 @@
+"""
+pytest entry point
+"""
+import pytest
+
+if __name__ == "__main__":
+    raise SystemExit(pytest.main())