[CM2] Link categories and collections in entry fields

A breadth-first search is done to match the shallowest entry first.

So, if these entries exist:
- demos/geocentric-universe
- demos/css/geocentric-universe
- demos/css/css-only/geocentric-universe

The following front-matter:
---
favoriteDemo: +demos/geocentric-universe
bestDemo: +demos/css/geocentric-universe
crazyDemo: +demos/css/css-only/geocentric-universe
---

would result in exactly what it looks like it should.

In the earlier code, subcategories were not being traversed and also a
bug (links.categorySlug) meant that it always looked at
collection.posts, and never actually considered the categorySlug.

With this update it's also fixed.

And categories and collections can now also be linked through
front-matter fields.

Author: scriptype

src/compiler/contentModel2/index.js

@@ -24,11 +24,36 @@ const parseLink = (value) => {
 }
 
 const findLinkedEntry = (contentModel, link) => {
-  const collection = contentModel.collections.find(c => c.slug.match(new RegExp(link.collectionSlug, 'i')))
-  const container = link.categorySlugs[0] ? // TODO: Handle subcategories
-    collection.categories.find(c => c.slug.match(new RegExp(link.categorySlug, 'i'))) || collection :
-    collection
-  return container.posts.find(p => p.slug.match(new RegExp(link.entrySlug, 'i')))
+  const collectionSlugRe = new RegExp(link.collectionSlug, 'i')
+  const collection = contentModel.collections.find(c => c.slug.match(collectionSlugRe))
+  let container = collection
+
+  for (const categorySlug of link.categorySlugs) {
+    const categorySlugRe = new RegExp(categorySlug, 'i')
+    const category = container.categories?.find(c => c.slug.match(categorySlugRe))
+    if (!category) {
+      break
+    }
+    container = category
+  }
+
+  const entrySlugRe = new RegExp(link.entrySlug, 'i')
+  const queue = [container]
+  while (queue.length > 0) {
+    const node = queue.shift()
+    if (node.slug?.match(entrySlugRe)) {
+      return node
+    }
+    const match = node.levelPosts?.find(p => p.slug.match(entrySlugRe))
+    if (match) {
+      return match
+    }
+    if (node.categories) {
+      queue.push(...node.categories)
+    }
+  }
+
+  return undefined
 }
 
 const linkBack = (post, entry, key) => {

src/compiler/contentModel2/models/collection/index.js

@@ -105,13 +105,15 @@ module.exports = function Collection(settings = defaultSettings, contentTypes =
         if (childModels.category.draftCheck(uncategorizedPost)) {
           defaultCategory.levelPosts.push(uncategorizedPost)
           defaultCategory.posts.push(uncategorizedPost)
+          tree.levelPosts.push(uncategorizedPost)
           tree.posts.push(uncategorizedPost)
         }
       }
 
       const tree = {
         categories: [],
         posts: [],
+        levelPosts: [],
         attachments: [],
         facets: []
       }