[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.
is it really internal while being at the root of the component ? this looks weird to me. Internal classes used for the building of ICU data only should probably be placed in a subnamespace, keeping the top-level of the component for the poublic API