fix(@ngtools/webpack): give higher priority to ivy-specific entry-points

When Ivy is enabled in an app, the app's dependencies are processed by
ngcc (the Angular compatibility compiler), which will generate new
Ivy-compatible entry-points in each package. To allow webpack find those
entry-points, it will add references to them in `package.json`, named
after the original property with the `_ivy_ngcc` suffix. So, for
example, `es2015` and `module` will become `es2015_ivy_ngcc` and
`module_ivy_ngcc`.

Previously, the `mainFields` passed to webpack would give higher
priority to an Ivy entry-point compared to the corresponding non-ivy
entry-point but not compared to other entry-points. For example,
`es2015, module` would becode
`es2015_ivy_ngcc, es2015, module_ivy_ngcc, module`. This could mean that
`es2015` would be preferred over `module_ivy_ngcc`, even though the
former is probably not Ivy-ready. Generally, if `es2015` exists and has
a higher priority than `module`, then `es2015_ivy_ngcc` would be
generated before (or instead of) `module_ivy_ngcc`, but this can be
changed using an ngcc configuration file (`ngcc.config.js`).

This commit fixes this by ensuring that any `_ivy_ngcc` entry-point is
considered before any of the original entry-points in the `mainFields`
list.

NOTE:
While it is possible that a format is Ivy-compatible without bhaving an
`_ivy_ngcc` suffix (e.g. if the user does a manual ngcc pass without
`--create-ivy-entry-points`), it is quite unlikely to happen and even if
it does, choosing a different format should be OK.

(cherry picked from commit c7c574a)

packages/ngtools/webpack/src/angular_compiler_plugin.ts

@@ -911,18 +911,18 @@ export class AngularCompilerPlugin {
         // When Ivy is enabled we need to add the fields added by NGCC
         // to take precedence over the provided mainFields.
         // NGCC adds fields in package.json suffixed with '_ivy_ngcc'
-        // Example: module -> module__ivy_ngcc
+        // Example: module -> module_ivy_ngcc
         // tslint:disable-next-line:no-any
         (compiler as any).resolverFactory.hooks.resolveOptions
           .for('normal')
           // tslint:disable-next-line:no-any
           .tap('WebpackOptionsApply', (resolveOptions: any) => {
-            const mainFields = (resolveOptions.mainFields as string[])
-              .map(f => [`${f}_ivy_ngcc`, f]);
+            const originalMainFields: string[] = resolveOptions.mainFields;
+            const ivyMainFields = originalMainFields.map(f => `${f}_ivy_ngcc`);
 
             return {
               ...resolveOptions,
-              mainFields: flattenArray(mainFields),
+              mainFields: [...ivyMainFields, ...originalMainFields],
             };
           });
       }