Issue #3529633: Explain Unknown parsers more clearly

Closes #3529633

src/Annotation/ComposerPackage.php

@@ -48,12 +48,13 @@ class ComposerPackage extends InstallableLibrary {
     if (!isset($this->availableVersions)) {
       $this->availableVersions = [];
       $id = $this->getId();
-      // To ensure we have the latest versions at all times, use the
-      // https://repo.packagist.org/p/[vendor]/[package].json URL which are
-      // static files and not cached.
-      $json = $this->requestJson(sprintf('https://repo.packagist.org/p/%s.json', $id));
+      // Get latest versions from the API.
+      // See https://packagist.org/apidoc#get-package-data.
+      $json = $this->requestJson(sprintf('https://repo.packagist.org/p2/%s.json', $id));
       if (!empty($json['packages'][$id])) {
-        $this->availableVersions = array_keys($json['packages'][$id]);
+        foreach ($json['packages'][$id] as $package) {
+          $this->availableVersions[] = $package['version'];
+        }
       }
     }
     return $this->availableVersions;