Export types of builtin fixture for type annotations

In order to allow users to type annotate fixtures they request, the
types need to be imported from the `pytest` namespace. They are/were
always available to import from the `_pytest` namespace, but that is
not guaranteed to be stable.

These types are only exported for the purpose of typing. Specifically,
the following are *not* public:

- Construction (`__init__`)
- Subclassing
- staticmethods and classmethods

We try to combat them being used anyway by:

- Marking the classes as `@final` when possible (already done).

- Not documenting private stuff in the API Reference.

- Using `_`-prefixed names or marking as `:meta private:` for private
  stuff.

- Adding a keyword-only `_ispytest=False` to private constructors,
  warning if False, and changing pytest itself to pass True. In the
  future it will (hopefully) become a hard error.

Hopefully that will be enough.

testing/test_cacheprovider.py

@@ -1156,7 +1156,7 @@ def test_gitignore(testdir):
     from _pytest.cacheprovider import Cache
 
     config = testdir.parseconfig()
-    cache = Cache.for_config(config)
+    cache = Cache.for_config(config, _ispytest=True)
     cache.set("foo", "bar")
     msg = "# Created by pytest automatically.\n*\n"
     gitignore_path = cache._cachedir.joinpath(".gitignore")
@@ -1178,7 +1178,7 @@ def test_does_not_create_boilerplate_in_existing_dirs(testdir):
         """
     )
     config = testdir.parseconfig()
-    cache = Cache.for_config(config)
+    cache = Cache.for_config(config, _ispytest=True)
     cache.set("foo", "bar")
 
     assert os.path.isdir("v")  # cache contents
@@ -1192,7 +1192,7 @@ def test_cachedir_tag(testdir):
     from _pytest.cacheprovider import CACHEDIR_TAG_CONTENT
 
     config = testdir.parseconfig()
-    cache = Cache.for_config(config)
+    cache = Cache.for_config(config, _ispytest=True)
     cache.set("foo", "bar")
     cachedir_tag_path = cache._cachedir.joinpath("CACHEDIR.TAG")
     assert cachedir_tag_path.read_bytes() == CACHEDIR_TAG_CONTENT