## Summary
This PR introduces a new mdtest option `system` that can either be
`in-memory` or `os`
where `in-memory` is the default.
The motivation for supporting `os` is so that we can write OS/system
specific tests
with mdtests. Specifically, I want to write mdtests for the module
resolver,
testing that module resolution is case sensitive.
## Test Plan
I tested that the case-sensitive module resolver test start failing when
setting `system = "os"`
## Summary
This PR adds support for an optional list of paths that should be
checked to `knot check`.
E.g. to only check the `src` directory
```sh
knot check src
```
The default is to check all files in the project but users can reduce
the included files by specifying one or multiple optional paths.
The main two challenges with adding this feature were:
* We now need to show an error when one of the provided paths doesn't
exist. That's why this PR now collects errors from the project file
indexing phase and adds them to the output diagnostics. The diagnostic
looks similar to ruffs (see CLI test)
* The CLI should pick up new files added to included folders. For
example, `knot check src --watch` should pick up new files that are
added to the `src` folder. This requires that we now filter the files
before adding them to the project. This is a good first step to
supporting `include` and `exclude`.
The PR makes two simplifications:
1. I didn't test the changes with case-insensitive file systems. We may
need to do some extra path normalization to support those well. See
https://github.com/astral-sh/ruff/issues/16400
2. Ideally, we'd accumulate the IO errors from the initial indexing
phase and subsequent incremental indexing operations. For example, we
should preserve the IO diagnostic for a non existing `test.py` if it was
specified as an explicit CLI argument until the file gets created and we
should show it again when the file gets deleted. However, this is
somewhat complicated because we'd need to track which files we revisited
(or were removed because the entire directory is gone). I considered
this too low a priority as it's worth dealing with right now.
The implementation doesn't support symlinks within the project but that
is the same as Ruff and is unchanged from before this PR.
Closes https://github.com/astral-sh/ruff/issues/14193
## Test Plan
Added CLI and file watching integration tests. Manually testing.
