internal: reduce crate interdependence

I don't think there's anything wrong with project_model depending on
proc_macro_api directly -- fundamentally, both are about gluing our pure
data model to the messy outside world.

However, it's easy enough to avoid the dependency, so why not.

As an additional consideration, `proc_macro_api` now pulls in `base_db`.
project_model should definitely not depend on that!

crates/project_model/src/lib.rs

@@ -50,8 +50,6 @@ pub use crate::{
     workspace::{CfgOverrides, PackageRoot, ProjectWorkspace},
 };
 
-pub use proc_macro_api::ProcMacroClient;
-
 #[derive(Debug, Clone, PartialEq, Eq, Hash, Ord, PartialOrd)]
 pub enum ProjectManifest {
     ProjectJson(ManifestPath),