RFR: 8346610: Make all imports conformant in the FFM API

[Per Minborg]

This PR proposes to clean up all the imports in the FFM lib (excluding tests).

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Commit messages:
 - Fix imports

Changes: https://git.openjdk.org/jdk/pull/22827/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22827&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346610
  Stats: 332 lines in 52 files changed: 144 ins; 154 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/22827.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22827/head:pull/22827

PR: https://git.openjdk.org/jdk/pull/22827