Add --root flag to specify project configuration

[cosinequanon]

For #110 there are use cases where projects might have multiple pyproject.toml files in the repo for various configurations but we want to store all the ufmt/usort/black configuration in one place. Currently there is no easy way to do this without calling ufmt directly on the root of the project which can be super expensive in large projects.

This adds a new flag so you can specify the root. This works well for the general configuration but I want to point out one area where it does not work correctly, specifically with excludes.

Say you have this set up

$ ls **
pyproject.toml

a:
a.py            pyproject.toml

b:
b.py

$ cat pyproject.toml
[tool.ufmt]
excludes = [
    "/a/",
]

all other files are empty. Then if you format the root you would expect only b to be formatted. If you format the a directory then a.py will get formatted currently as well.

$ ufmt --debug check .
DEBUG ufmt.core Checking /private/tmp/project/b/b.py
✨ 1 file already formatted ✨
$ ufmt --debug check a
DEBUG ufmt.core Checking /private/tmp/project/a/a.py
✨ 1 file already formatted ✨

works as expected. the issue is now with the new flag in this PR, specifying the root and formatting directory a doesn't do what we expect

$ ufmt --root=. --debug check a
DEBUG ufmt.core Checking /private/tmp/project/a/a.py
✨ 1 file already formatted ✨

The issue is that overriding the root here is insufficient, we would also need to tell Trailrunner about it too, see https://github.com/omnilib/trailrunner/blob/main/trailrunner/core.py#L170

So this flag will help with some configuration but it might be confusing in terms of the excludes unless we also patch the other library to allow you to set the root.

[cosinequanon]

@amyreese could you please take a look, thanks!

[amyreese]

Thank you, and sorry for the delay in reviewing this. I updated it to use the click.Path type for validating the values of --root, but otherwise looks great, and I appreciate your work on this.

re: trailrunner, if you're interested in a PR there to accept a root parameter to the Trailrunner constructor and plumb that through in µfmt, I'd be happy to review that as well.

Cheers! 🍻