Previously we were using an anonymous type `{specifier: string; qualifier: string;}`
throughout the code base. This commit gives this type a name and ensures it
is only defined in one place.
PR Close#25445
Now that we are using package.json properties to indicate which
entry-point format to compile, it turns out that we don't really
need to distinguish between flat and non-flat formats, unless we
are compiling `@angular/core`.
PR Close#29092