Path handling on non-case sensitive systems

[MichaReiser]

The project file discovery and the Files interner assume that two paths that only different in casing are two unique paths. However, this is not true on all file systems and it could result in Red Knot considering two files as being different even though they're the same on disk.

We should test how Red Knot handles paths with different casing on case-insensitive file systems (looking at you Windows). That also includes project file discovery.

Related to astral-sh/ruff#13691

[MichaReiser]

astral-sh/ruff#16521 makes the module resolver case sensitive. It also adds a test demonstrating where our watcher gets out of sync with the Files state on case-insensitive systems (the test is currently ignored because of it and fixing this PR should make this test pass)

The main outstanding issue is that Files is case sensitive but it doesn't use case-sensitive system calls. However, how we fix this depends on a UX question: Should paths passed to the Red Knot CLI or in configurations be case sensitive on all platforms or only on case-sensitve platforms. Depending on that, we may need to make Files case-sensitive or respect the platform's case sensitivity and we may have to do the same for all SystemPath methods that compare paths (starts_with, ends_with, equals, ...)