Single-package mode

[matklad]

Today rust-analyzer unconditionally operates on the whole workspace, even if you open a subfolder of the workspace with a single package. It might be reasonable to scope the work to a single package in such cases.

The tradeoff here is:

• when you open a single package, refactors don't work across the workspace
• but ra needs to cargo check a single package only, which is especially helpful if the overall build is broken

I haven't thought too deeply how this all should work exactly.

related user request: #10730

Relevant code:

https://github.com/rust-analyzer/rust-analyzer/blob/1ba9a924d7b161c52e605e157ee16d582e4a8684/crates/project_model/src/workspace.rs#L139-L145

https://github.com/rust-analyzer/rust-analyzer/blob/1ba9a924d7b161c52e605e157ee16d582e4a8684/crates/project_model/src/lib.rs#L84

[bjorn3]

It would also be nice to allow a subset of a workspace. For example in https://github.com/bytecodealliance/wasmtime I often only work on Cranelift which has all crates in the cranelift directory.