[code-reflection] Integrated: Create extern package

[Paul Sandoz]

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.

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

Commit messages:
 - Create extern package.

Changes: https://git.openjdk.org/babylon/pull/458/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=458&range=00
  Stats: 196 lines in 78 files changed: 29 ins; 42 del; 125 mod
  Patch: https://git.openjdk.org/babylon/pull/458.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/458/head:pull/458

PR: https://git.openjdk.org/babylon/pull/458