build: publish tree of files rather than FESMs (#18541)

* Remove now unnecessary portions of build.
* Add a compilePackageES5 method to build ES5 from sources
* Rework all package.json and rollup config files to new format
* Remove "extends" from tsconfig-build.json files and fixup compilation roots

PR Close #18541

packages/forms/public_api.ts

@@ -9,7 +9,7 @@
 /**
  * @module
  * @description
- * Entry point for all public APIs of the forms package.
+ * Entry point for all public APIs of this package.
  */
 export * from './src/forms';