[Intl] Improved bundle reader implementations

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR extracts bundle reader improvements from #9206.

The code is internal and used for resource bundle generation only, so I did not care about BC too much.
I'm going to remove this flag in 2.6, but I don't want anybody to rely on this code until I'm sure this class will stay as it is.