[code-reflection] Integrated: mkpoms cleanup + .gitignore target dirs

[Gary Frost]

Cleanup mkpoms script. 

Also added all intermediate maven target dirs to `.gitignores`

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Commit messages:
 - mkpoms cleanup + .gitignore target dirs

Changes: https://git.openjdk.org/babylon/pull/285/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=285&range=00
  Stats: 35 lines in 14 files changed: 20 ins; 3 del; 12 mod
  Patch: https://git.openjdk.org/babylon/pull/285.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/285/head:pull/285

PR: https://git.openjdk.org/babylon/pull/285