message: fix core loading
diff: import { lookup } from 'dns';
  -import { Extension, ExtensionManager } from '@azure-tools/extension';
  +import { Extension, ExtensionManager, Package } from '@azure-tools/extension';
  import { homedir } from 'os';
  import { dirname, join, resolve } from 'path';
  @@ -218,18 +218,27 @@ export async function selectVersion(requestedVersion: string, force: boolean, mi
  }
  let corePackageName = newCorePackage;
  - let pkg = await (await extensionManager).findPackage(newCorePackage, requestedVersion);
  - if (pkg) {
  - if (args.debug) {
  - console.log(`Selected package: ${pkg.name}@${pkg.version} => ${pkg.resolvedInfo.rawSpec} `);
  - }
  - } else {
  + let pkg: Package;
  + try {
  + // try the package
  + pkg = await (await extensionManager).findPackage(newCorePackage, requestedVersion);
  + } catch {
  + // fallback to old package name
  + try {
  pkg = await (await extensionManager).findPackage(oldCorePackage, requestedVersion);
  + } catch {
  + // no package found!
  + }
  if (!pkg) {
  - throw new Exception(`Unable to find a valid AutoRest core package for '${requestedVersion}'.`);
  + throw new Exception(`Unable to find a valid AutoRest core package '${newCorePackage}' @ '${requestedVersion}'.`);
  }
  corePackageName = oldCorePackage;
  }
  + if (pkg) {
  + if (args.debug) {
  + console.log(`Selected package: ${pkg.name}@${pkg.version} => ${pkg.resolvedInfo.rawSpec} `);
  + }
  + }
  // pkg.version == the actual version
  // check if it's installed already.
  