[upstream] rust-analyzer attaches, but doesn't work for second standalone file

To reproduce:

- In a non-project, open `foo.rs`
  - rust-analyzer attaches in standalone/detached mode
  - hover, completions, etc. work
- `:e bar.rs`
  - rust-analyzer attaches
  - hover, completions, etc. don't work work

A workaround was implemented in #189.
But this has the issue that separate rust-analyzer processes are spawned for each file.