Previously, the transitive scopes of an NgModuleDef were computed
during execution of the @NgModule decorator. This meant that JIT-
compiled modules could only import other JIT-compiled modules, as
the import mechanism relied on the calculation of transitive scopes
to already have happened for the imported module.
This change moves computation of transitive scopes to a function
`transitiveScopesFor` (and makes it lazy). This opens the door for
AOT -> JIT or JIT -> AOT imports, as transitive scopes for AOT
modules can be calculated when needed by JIT, and AOT modules can
also write expressions that call `transitiveScopesFor` when
importing a JIT-compiled module.
PR Close#24334