"FileNotFoundError" due to missing "schema.json" after installation

[Delgan]

Hi.

When testing using master branch, I noticed an error at runtime after installing pytest-mypy-plugins.
This can be easily reproduced by cloning this repository, installing the plugin pip install ., then running pytest on a project using this lib.

$ pytest tests/typesafety
Traceback (most recent call last):
  File "/home/delgan/pytest-mypy-plugins/env/bin/pytest", line 8, in <module>
    sys.exit(console_main())
             ^^^^^^^^^^^^^^
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/_pytest/config/__init__.py", line 192, in console_main
    code = main()
           ^^^^^^
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/_pytest/config/__init__.py", line 150, in main
    config = _prepareconfig(args, plugins)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/_pytest/config/__init__.py", line 331, in _prepareconfig
    config = pluginmanager.hook.pytest_cmdline_parse(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/pluggy/_hooks.py", line 493, in __call__
    return self._hookexec(self.name, self._hookimpls, kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/pluggy/_manager.py", line 115, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/pluggy/_callers.py", line 130, in _multicall
    teardown[0].send(outcome)
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/_pytest/helpconfig.py", line 104, in pytest_cmdline_parse
    config: Config = outcome.get_result()
                     ^^^^^^^^^^^^^^^^^^^^
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/pluggy/_result.py", line 114, in get_result
    raise exc.with_traceback(exc.__traceback__)
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/pluggy/_callers.py", line 77, in _multicall
    res = hook_impl.function(*args)
          ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/_pytest/config/__init__.py", line 1075, in pytest_cmdline_parse
    self.parse(args)
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/_pytest/config/__init__.py", line 1425, in parse
    self._preparse(args, addopts=addopts)
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/_pytest/config/__init__.py", line 1305, in _preparse
    self.pluginmanager.load_setuptools_entrypoints("pytest11")
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/pluggy/_manager.py", line 398, in load_setuptools_entrypoints
    plugin = ep.load()
             ^^^^^^^^^
  File "/usr/lib/python3.11/importlib/metadata/__init__.py", line 202, in load
    module = import_module(match.group('module'))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1204, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1176, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1147, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 690, in _load_unlocked
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/_pytest/assertion/rewrite.py", line 178, in exec_module
    exec(co, module.__dict__)
  File "/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/pytest_mypy_plugins/collect.py", line 34, in <module>
    SCHEMA = json.loads((pathlib.Path(__file__).parent / "schema.json").read_text("utf8"))
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/pathlib.py", line 1058, in read_text
    with self.open(mode='r', encoding=encoding, errors=errors) as f:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/pathlib.py", line 1044, in open
    return io.open(self, mode, buffering, encoding, errors, newline)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/home/delgan/pytest-mypy-plugins/env/lib/python3.11/site-packages/pytest_mypy_plugins/schema.json'

The recently introduced "schema.json" in #127 is not embedded in the installed folder, causing an error at runtime.

It's an easy fix, I'll open a PR.