rustdoc-json: Items who'se original path isn't visiable doen't appear in Crate::paths

[aDotInTheVoid]

With this code:

#![feature(no_core)]
#![no_core]

mod inner {
    pub struct Public;
}

pub use inner::Public;

We produce the following JSON (redacted for clarity):

{
  "crate_version": null,
  "external_crates": {},
  "format_version": 37,
  "includes_private": false,
  "index": {
    "0": {
      "attrs": [],
      "docs": null,
      "inner": {"struct": {"generics": {"params": [], "where_predicates": []}, "impls": [], "kind": "unit"}},
      "name": "Public",
      "visibility": "public"
    },
    "2": {
      "attrs": [],
      "docs": null,
      "inner": {"use": {"id": 0, "is_glob": false, "name": "Public", "source": "inner::Public"}},
      "name": null,
      "visibility": "public"
    },
    "3": {
      "attrs": ["#![feature(no_core)]", "#![no_core]"],
      "docs": null,
      "inner": {"module": {"is_crate": true, "is_stripped": false, "items": [2]}},
      "name": "simple_private",
      "visibility": "public"
    }
  },
  "paths": {"3": {"crate_id": 0, "kind": "module", "path": ["simple_private"]}},
  "root": 3
}

There's no path entry for 0, but there should be.

---

Found while looking at #134880.

We should have the jsondoclint validator check this, something like

diff --git a/src/tools/jsondoclint/src/validator.rs b/src/tools/jsondoclint/src/validator.rs
index f7c752033c5..73aa86d9c90 100644
--- a/src/tools/jsondoclint/src/validator.rs
+++ b/src/tools/jsondoclint/src/validator.rs
@@ -303,7 +303,12 @@ fn check_path(&mut self, x: &'a Path, kind: PathKind) {
             PathKind::Trait => self.add_trait_or_alias_id(&x.id),
             PathKind::Type => self.add_type_id(&x.id),
         }
-        if let Some(args) = &x.args {
+
+        if !self.krate.paths.contains_key(&x.id) {
+            panic!("no $.paths entry for {x:?}");
+        }
+
+        if let Some(args) = &*x.args {
             self.check_generic_args(&**args);
         }
     }