refactor: add explicit types for exports relying on inferred call return type (#61312)

As part of the Bazel toolchain migration we noticed that implicit types
generated by the TypeScript compiler sometimes end up referencing types
from other packages (i.e. cross-package imports).

These imports currently work just because the Bazel `ts_library` and
`ng_module` rules automatically inserted a `<amd-module
name="@angular/x" />` into `.d.ts` of packages. This helped TS figure
out how to import a given file. Notably this is custom logic that is not
occuring in vanilla TS or Angular compilations—so we will drop this
magic as part of the toolchain cleanup!

To improve code quality and keep the existing behavior working, we are
doing the following:

- adding a lint rule that reduces the risk of such imports breaking. The
  failure scenario without the rule is that API goldens show unexpected
  diffs, and types might be duplicated in a different package!

- keeping the `<amd-module` headers, but we manually insert them into
  the package entry-points. This should ensure we don't regress
  anywhere; while we also improved general safety around this above.

Long-term, isolated declarations or a lint rule from eslint-typescript
can make this even more robust.

PR Close #61312

packages/platform-server/test/hydration_utils.ts

@@ -42,7 +42,7 @@ export const EMPTY_TEXT_NODE_COMMENT = 'ngetn';
 export const TEXT_NODE_SEPARATOR_COMMENT = 'ngtns';
 
 export const SKIP_HYDRATION_ATTR_NAME = 'ngSkipHydration';
-export const SKIP_HYDRATION_ATTR_NAME_LOWER_CASE = SKIP_HYDRATION_ATTR_NAME.toLowerCase();
+export const SKIP_HYDRATION_ATTR_NAME_LOWER_CASE: string = SKIP_HYDRATION_ATTR_NAME.toLowerCase();
 
 export const TRANSFER_STATE_TOKEN_ID = '__nghData__';