nightly

Auto merge of #17287 - Veykril:sysroot-encode-empty, r=Veykril

Allow sysroots to only consist of the source root dir

Fixes https://github.com/rust-lang/rust-analyzer/issues/17159

This PR encodes the `None` case of an optional sysroot into `Sysroot` itself. This simplifies a lot of things and allows us to have sysroots that consist of nothing, only standard library sources, everything but the standard library sources or everything. This makes things a lot more flexible. Additionally, this removes the workspace status bar info again, as it turns out that that can be too much information for the status bar to handle (this is better rendered somewhere else, like in the status view).