[Intl] Improved bundle reader implementations
bug #11907 [Intl] Improved bundle reader implementations (webmozart)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
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c3cce5c [Intl] Improved bundle reader implementations