gpui: Further docs refinement & moved some reexports into 'private' module (#3935)

This commit mostly fixes invalid URLs in docstrings. It also
encapsulates crates we reexport (serde stuff + linkme) into a public
module named "private" in order to reduce the API surfaced through docs.
Moreover, I fixed up a bunch of crates that were pulling serde_json in
through gpui explicitly instead of using Cargo manifest.

Release Notes:
- N/A

crates/client/src/client.rs

@@ -15,8 +15,8 @@ use futures::{
     TryFutureExt as _, TryStreamExt,
 };
 use gpui::{
-    actions, serde_json, AnyModel, AnyWeakModel, AppContext, AsyncAppContext, Model,
-    SemanticVersion, Task, WeakModel,
+    actions, AnyModel, AnyWeakModel, AppContext, AsyncAppContext, Model, SemanticVersion, Task,
+    WeakModel,
 };
 use lazy_static::lazy_static;
 use parking_lot::RwLock;
@@ -25,6 +25,7 @@ use rand::prelude::*;
 use rpc::proto::{AnyTypedEnvelope, EntityMessage, EnvelopedMessage, PeerId, RequestMessage};
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
+use serde_json;
 use settings::Settings;
 use std::{
     any::TypeId,