refactor: source yarn versions from npm not github

lib/manager/npm/extract/index.js

@@ -116,7 +116,7 @@ async function extractDependencies(content, packageFile, config) {
             if (depName === 'node') {
               dep.purl = 'pkg:github/nodejs/node?clean=true';
             } else if (depName === 'yarn') {
-              dep.purl = 'pkg:github/yarnpkg/yarn?clean=true';
+              dep.purl = 'pkg:npm/yarn';
               dep.commitMessageTopic = 'Yarn';
             } else if (depName === 'npm') {
               dep.purl = 'pkg:npm/npm';

test/manager/npm/extract/__snapshots__/index.spec.js.snap

@@ -70,7 +70,7 @@ Object {
       "currentValue": "disabled",
       "depName": "yarn",
       "depType": "engines",
-      "purl": "pkg:github/yarnpkg/yarn?clean=true",
+      "purl": "pkg:npm/yarn",
       "skipReason": "unknown-version",
       "versionScheme": "semver",
     },