Two `test_editable` failures with Meson master due to path separator changes

[rgommers]

From this CI log and #747 (comment):

FAILED tests/test_editable.py::test_mesonpy_meta_finder - AssertionError: assert 'D:/a/meson-python/meson-python/tests/packages/complex/complex\\__init__.py' == 'D:\\a\\meson-python\\meson-python\\tests\\packages\\complex\\complex\\__init__.py'
  
  - D:\a\meson-python\meson-python\tests\packages\complex\complex\__init__.py
  ?   ^ ^            ^            ^     ^        ^       ^
  + D:/a/meson-python/meson-python/tests/packages/complex/complex\__init__.py
  ?   ^ ^            ^            ^     ^        ^       ^
FAILED tests/test_editable.py::test_importlib_resources - AssertionError: assert 'D:/a/meson-python/meson-python/tests/packages/simple/__init__.py' == 'D:\\a\\meson-python\\meson-python\\tests\\packages\\simple\\__init__.py'

Comments from gh-747:

mesonbuild/meson#12194 probably

Most likely. That patch changes the format of the introspection data used by meson-python. So far meson-python assumes that the introspection data uses native path separators. Now, some data uses native path separators, and some uses POSIX path separators independently of the platform. For supporting Meson version before and after the change, meson-python will need to normalize all paths read from introspection data.

[rgommers]

For supporting Meson version before and after the change, meson-python will need to normalize all paths read from introspection data.

From the test failures I deduce that the damage is small and this won't be a regression on Windows beyond editable installs - but I'll go ahead and verify that.