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/http/testing/index.ts

@@ -12,4 +12,4 @@
  * Entry point for all public APIs of the http/testing package.
  */
 
-export * from './src/testing';
+export * from './public_api';