Avoid blocking the main loop when editing Cargo.toml

I've noticed a bunch of "main loop too long" warnings in console when
typing in Cargo.toml. Profiling showed that the culprit is `rustc
--print cfg` call.

I moved it to the background project loading phase, where it belongs.
This highlighted a problem: we generally use single `cfg`, while it
really should be per crate.

crates/project_model/src/lib.rs

@@ -5,6 +5,7 @@ mod cfg_flag;
 mod project_json;
 mod sysroot;
 mod workspace;
+mod rustc_cfg;
 
 use std::{
     fs::{read_dir, ReadDir},