Tree-Shakeable Tokens - #22005

This is needed so the rules produce metadata.json files, which is essential
for building compiler/integration tests with Bazel.

PR Close #22005

…at runtime (#22005)

All of the providers in a module get compiled into a module definition in the
factory file. Some of these providers are for the actual module types, as those
are available for injection in Angular. For tree-shakeable tokens, the runtime
needs to be able to distinguish which modules are present in an injector.

This change adds a NodeFlag which tags those module providers for later
identification.

PR Close #22005

This commit bundles 3 important changes, with the goal of enabling tree-shaking
of services which are never injected. Ordinarily, this tree-shaking is prevented
by the existence of a hard dependency on the service by the module in which it
is declared.

Firstly, @Injectable() is modified to accept a 'scope' parameter, which points
to an @NgModule(). This reverses the dependency edge, permitting the module to
not depend on the service which it "provides".

Secondly, the runtime is modified to understand the new relationship created
above. When a module receives a request to inject a token, and cannot find that
token in its list of providers, it will then look at the token for a special
ngInjectableDef field which indicates which module the token is scoped to. If
that module happens to be in the injector, it will behave as if the token
itself was in the injector to begin with.

Thirdly, the compiler is modified to read the @Injectable() metadata and to
generate the special ngInjectableDef field as part of TS compilation, using the
PartialModules system.

Additionally, this commit adds several unit and integration tests of various
flavors to test this change.

PR Close #22005