build(bazel): check for renderer flag in `ng_module()` (#37529)

This checks for a Bazel flag in `ng_module()` in the `_renderer` attribute
which specifies the renderer to use for the build.

The main advantage of this flag is that it can be overridden with [Bazel
transitions](https://docs.bazel.build/versions/master/skylark/config.html),
giving much more flexibility for migrating individual applications in a
Bazel workspace to Ivy.

This flag is not intended to replace `--config ivy` or
`--define angular_ivy_enabled=True` (although it technically could). As a
result, this flag is not and will not actually be used anywhere in the
`angular/angular` repo. Instead, a `string_flag()` is provided  internally
which sets the renderer via a transition. See http://cl/315749946.

Note that this does **not** introduce a dependency on Skylib for
`angular/angular`. The dependency isn't actually necessary because
`BuildSettingInfo` is not used externally anyways. By doing this, it is not
necessary for downstream, external workspaces to depend on Skylib.

PR Close #37529

packages/bazel/src/external.bzl

@@ -35,6 +35,11 @@ compile_ts = _compile_ts
 DEPS_ASPECTS = _DEPS_ASPECTS
 ts_providers_dict_to_struct = _ts_providers_dict_to_struct
 
+# Should be defined as `BuildSettingInfo` from Skylib, but a dependency on
+# Skylib is not necessary here because this is only used in google3 where Skylib
+# is loaded differently anyways where this file is overridden.
+BuildSettingInfo = provider(doc = "Not used outside google3.")
+
 DEFAULT_API_EXTRACTOR = "@npm//@angular/bazel/bin:api-extractor"
 DEFAULT_NG_COMPILER = "@npm//@angular/bazel/bin:ngc-wrapped"
 DEFAULT_NG_XI18N = "@npm//@angular/bazel/bin:xi18n"

packages/bazel/src/ng_module.bzl

@@ -7,6 +7,7 @@
 
 load(
     ":external.bzl",
+    "BuildSettingInfo",
     "COMMON_ATTRIBUTES",
     "COMMON_OUTPUTS",
     "DEFAULT_API_EXTRACTOR",
@@ -36,6 +37,15 @@ def is_ivy_enabled(ctx):
       Boolean, Whether the ivy compiler should be used.
     """
 
+    # Check the renderer flag to see if Ivy is enabled.
+    # This is intended to support a transition use case for google3 migration.
+    # The `_renderer` attribute will never be set externally, but will always be
+    # set internally as a `string_flag()` with the allowed values of:
+    # "view_engine" or "ivy".
+    if ((hasattr(ctx.attr, "_renderer") and
+         ctx.attr._renderer[BuildSettingInfo].value == "ivy")):
+        return True
+
     # TODO(josephperott): Remove after ~Feb 2020, to allow local script migrations
     if "compile" in ctx.var and ctx.workspace_name == "angular":
         fail(