RFR: 8332497: javac crashes when annotation processing runs on program with module imports

[Evemose]

Fix is pretty simple: visitModuleImport in com.sun.tools.javac.tree.TreeScanner has notbeen overriden, so defaulted to Visitor::visitModuleImport, which forwards to Visitor::visitTree, which is also not overriden, and, therefore, threw AssertionError. 

PS: Im not even sure how it worked before without crashing, seems like there is some intermidiate implementation between this TreeScanner and actual scanners because otherwise it should have resultedin compile error the moment it encounter module importin any visitor

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Commit messages:
 - Add visitModuleImport override to com.sun.tools.javac.tree.TreeScanner

Changes: https://git.openjdk.org/jdk/pull/19292/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19292&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332497
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19292.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19292/head:pull/19292

PR: https://git.openjdk.org/jdk/pull/19292