RFR: 8306055: Add a built-in Catalog to JDK XML module [v2]

[Joe Wang]

> Implement the built-in Catalog.

Joe Wang has updated the pull request incrementally with one additional commit since the last revision:

  remove J2SE directory; add note explaining how DTDs are resolved

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16719/files
  - new: https://git.openjdk.org/jdk/pull/16719/files/f52f8ad6..c1059a84

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16719&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16719&range=00-01

  Stats: 8 lines in 4 files changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16719.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16719/head:pull/16719

PR: https://git.openjdk.org/jdk/pull/16719