feat: dynamic rollup configuration for esm flattening (#395)

BREAKING CHANGE:
`lib.externals` has been removed in favour of `lib.umdModuleIds` which is now just used to provide the UMD module identifiers of external dependencies.
By default, all dependencies are now treated as externals and thus are not embedded in the final bundle.
If a dependency should be embedded in the distributables, it needs to be explicity added to `lib.embedded`.
Please consult the updated README on migrating your package confguration from `lib.externals` to `lib.umdModuleIds` and `lib.embedded`.

README.md

@@ -199,6 +199,44 @@ Valid values: `none` or `inline`.
 }
 ```
 
+#### Third-party dependencies: external and embedded dependencies
+
+By default, dependencies of a library are treated as external dependencies and thus are not embedded in the final bundle.
+In most cases, you should expect that third-party dependencies will be part of the [`peerDependencies` of your distributables](https://nodejs.org/uk/blog/npm/peer-dependencies/).
+
+However, if you want to embed a dependency into the distributable bundle you are able to do so by adding the dependency in the `embedded` section like so:
+
+***HEADS UP***: embedding a dependency will result in you shipping the dependency's source code to your users!
+
+```json
+{
+  "$schema": "../../../src/ng-package.schema.json",
+  "lib": {
+    "embedded": [
+      "lodash",
+      "date-fns"
+    ]
+  }
+}
+```
+
+By default, ng-packagr will treat dependencies as external dependencies.
+When writing the [UMD bundle](https://github.com/umdjs/umd), ng-packagr does its best to provide common default values for the UMD module identifiers and `rollup` will also do its best to guess the module ID of an external dependency.
+Even then, you should make sure that the UMD module identifiers of the external dependencies are correct.
+In case ng-packagr doesn't provide a default and rollup is unable to guess the correct identifier, you should explicitly provide the module identifier by using `umdModuleIds` in the library's package file section like so:
+
+```json
+{
+  "$schema": "../../../src/ng-package.schema.json",
+  "lib": {
+    "umdModuleIds": {
+      "lodash" : "_",
+      "date-fns" : "DateFns",
+    }
+  }
+}
+```
+
 #### React loves Angular, Angular loves React
 
 What if I want to use React Components in Angular?
@@ -211,7 +249,7 @@ construct them, you can set the `jsx` flag for your library through `ng-package.
   "$schema": "../../../src/ng-package.schema.json",
   "lib": {
     "entryFile": "public_api.ts",
-    "externals": {
+    "umdModuleIds": {
       "react": "React",
       "react-dom": "ReactDOM"
     },
@@ -222,7 +260,7 @@ construct them, you can set the `jsx` flag for your library through `ng-package.
 
 The `jsx` flag will accept what the corresponding `tsconfig` accepts, more information [in the TypeScript Handbook chaper on JSX](https://www.typescriptlang.org/docs/handbook/jsx.html).
 
-Note: Don't forget to include `react` and `react-dom` in your `externals` so that you're not bundling those dependencies!
+Note: Don't forget to include `react` and `react-dom` in your `umdModuleIds` so that you're not bundling those dependencies!
 
 
 ## Further documentation