monkeypatch.syspath_prepend: call fixup_namespace_packages

Without the patch the test fails as follows:

            # Prepending should call fixup_namespace_packages.
            monkeypatch.syspath_prepend("world")
    >       import ns_pkg.world
    E       ModuleNotFoundError: No module named 'ns_pkg.world'

changelog/4980.feature.rst

@@ -0,0 +1 @@
+``monkeypatch.syspath_prepend`` calls ``pkg_resources.fixup_namespace_packages`` to handle namespace packages better.

src/_pytest/monkeypatch.py

@@ -262,10 +262,15 @@ class MonkeyPatch(object):
 
     def syspath_prepend(self, path):
         """ Prepend ``path`` to ``sys.path`` list of import locations. """
+        from pkg_resources import fixup_namespace_packages
+
         if self._savesyspath is None:
             self._savesyspath = sys.path[:]
         sys.path.insert(0, str(path))
 
+        # https://github.com/pypa/setuptools/blob/d8b901bc/docs/pkg_resources.txt#L162-L171
+        fixup_namespace_packages(str(path))
+
     def chdir(self, path):
         """ Change the current working directory to the specified path.
         Path can be a string or a py.path.local object.

testing/test_monkeypatch.py

@@ -437,3 +437,28 @@ def test_context():
         m.setattr(functools, "partial", 3)
         assert not inspect.isclass(functools.partial)
     assert inspect.isclass(functools.partial)
+
+
+def test_syspath_prepend_with_namespace_packages(testdir, monkeypatch):
+    for dirname in "hello", "world":
+        d = testdir.mkdir(dirname)
+        ns = d.mkdir("ns_pkg")
+        ns.join("__init__.py").write(
+            "__import__('pkg_resources').declare_namespace(__name__)"
+        )
+        lib = ns.mkdir(dirname)
+        lib.join("__init__.py").write("def check(): return %r" % dirname)
+
+    monkeypatch.syspath_prepend("hello")
+    import ns_pkg.hello
+
+    assert ns_pkg.hello.check() == "hello"
+
+    with pytest.raises(ImportError):
+        import ns_pkg.world
+
+    # Prepending should call fixup_namespace_packages.
+    monkeypatch.syspath_prepend("world")
+    import ns_pkg.world
+
+    assert ns_pkg.world.check() == "world"