RFR: 7903156: remove jopt-simple dependency from jextract

[Athijegannathan Sundararajan]

removed jopt-simple dependency as well as moditect grade plugin dependency. Piggybacking build.gradle change to enforce 'jtreg_home' check in jtreg task's doFirst block.

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Commit messages:
 - 7903156: remove jopt-simple dependency from jextract

Changes: https://git.openjdk.java.net/jextract/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=19&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903156
  Stats: 277 lines in 7 files changed: 178 ins; 62 del; 37 mod
  Patch: https://git.openjdk.java.net/jextract/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/19/head:pull/19

PR: https://git.openjdk.java.net/jextract/pull/19