DEV: Support colocation of component templates in plugins (#18598)

This allows plugins to colocate component JS and HBS under `/plugins/{name}/assets/javascripts/discourse/components`.

`discourse-presence` is updated to use this new pattern, which also serves as an integration test for this part of the build pipeline.

app/assets/javascripts/discourse-plugins/colocated-template-compiler.js

@@ -0,0 +1,19 @@
+const ColocatedTemplateProcessor = require("ember-cli-htmlbars/lib/colocated-broccoli-plugin");
+
+module.exports = class DiscoursePluginColocatedTemplateProcessor extends (
+  ColocatedTemplateProcessor
+) {
+  detectRootName() {
+    const entries = this.currentEntries().filter((e) => !e.isDirectory());
+
+    const path = entries[0]?.relativePath;
+
+    const match = path?.match(
+      /^discourse\/plugins\/(?<name>[^/]+)\/discourse\//
+    );
+
+    if (match) {
+      return `discourse/plugins/${match.groups.name}/discourse`;
+    }
+  }
+};

app/assets/javascripts/discourse-plugins/index.js

@@ -7,6 +7,7 @@ const mergeTrees = require("broccoli-merge-trees");
 const fs = require("fs");
 const concat = require("broccoli-concat");
 const RawHandlebarsCompiler = require("discourse-hbr/raw-handlebars-compiler");
+const DiscoursePluginColocatedTemplateProcessor = require("./colocated-template-compiler");
 
 function fixLegacyExtensions(tree) {
   return new Funnel(tree, {
@@ -169,6 +170,8 @@ module.exports = {
     tree = namespaceModules(tree, pluginName);
 
     tree = RawHandlebarsCompiler(tree);
+
+    tree = new DiscoursePluginColocatedTemplateProcessor(tree);
     tree = this.compileTemplates(tree);
 
     tree = this.processedAddonJsFiles(tree);