Fix package name

[thewtex]

The package name in pyproject.toml is now xarray-dataclass but the package directory is still src/xarray_dataclasses. This will cause conflicts and issues when both the xarray-dataclasses and xarray-dataclass packages are installed.

[melonora]

hmmm would there be a case in which a user wants both libraries installed? I can change this but then either I would bump the version to 3.0.0 or do a post release. I could have changed it already but didn't in order to make it easier for users to just switch

[thewtex]

@melonora thanks.

This is to help the use case where a user has a dependency on multiple packages where one package depends on xarray-dataclass and another depends on xarray-dataclasses.