RFR: 8344708: Compiler Implementation of Module Import Declarations

[Jan Lahoda]

This is a patch to finalize the module imports feature. Please see:
https://bugs.openjdk.org/browse/JDK-8344700

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Commit messages:
 - Cleanup.
 - Cleanup.
 - Finalizing module imports.

Changes: https://git.openjdk.org/jdk/pull/23801/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23801&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344708
  Stats: 113 lines in 15 files changed: 9 ins; 71 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/23801.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23801/head:pull/23801

PR: https://git.openjdk.org/jdk/pull/23801