fix(@angular/cli): support update migration packages with no entry points

Some schematic only packages may not have entry points defined (`main`/`exports`).  These type of packages will now be correctly resolved when attempting to locate update migrations.

Fixes #20032

(cherry picked from commit 3d99468)

Author: clydin

packages/angular/cli/commands/update-impl.ts

@@ -665,7 +665,21 @@ export class UpdateCommand extends Command<UpdateCommandSchema> {
           `Resolving migration package '${migration.package}' from '${this.context.root}'...`,
         );
         try {
-          packagePath = require.resolve(migration.package, { paths: [this.context.root] });
+          try {
+            packagePath = path.dirname(
+              // This may fail if the `package.json` is not exported as an entry point
+              require.resolve(path.join(migration.package, 'package.json'), {
+                paths: [this.context.root],
+              }),
+            );
+          } catch (e) {
+            if (e.code === 'MODULE_NOT_FOUND') {
+              // Fallback to trying to resolve the package's main entry point
+              packagePath = require.resolve(migration.package, { paths: [this.context.root] });
+            } else {
+              throw e;
+            }
+          }
         } catch (e) {
           if (e.code === 'MODULE_NOT_FOUND') {
             logVerbose(e.toString());