[code-reflection] Integrated: Create extern package

[Maurizio Cimadamore]

On Mon, 23 Jun 2025 21:21:22 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

> Consolidate packages `parser` and `writer` into an `extern` package. The classes directly under the `dialect` package will be moved to be directly under the `extern` package. Thereby we consolidate all externalization/reconstitution functionality under the same package.

New organization looks neat. Finding a good name for `extern` is hard (I thought about `io` but -- meh). Let's keep it for now.

-------------

Marked as reviewed by mcimadamore (Reviewer).

PR Review: https://git.openjdk.org/babylon/pull/458#pullrequestreview-2951588786