[jdk19] RFR: 8291512: Snippetize modules API examples [v2]

[Mark Reinhold]

> The specification of the modules API contains several non-normative code examples. We should convert them to use snippets, per [JEP 413](https://openjdk.org/jeps/413).

Mark Reinhold has updated the pull request incrementally with one additional commit since the last revision:

  Fix header in ModuleLayer

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Changes:
  - all: https://git.openjdk.org/jdk19/pull/156/files
  - new: https://git.openjdk.org/jdk19/pull/156/files/22efa8cb..4a79fe11

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk19&pr=156&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk19&pr=156&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk19/pull/156.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/156/head:pull/156

PR: https://git.openjdk.org/jdk19/pull/156