RFR: 8296734: JarVerifier:: mapSignersToCodeSource should cache in map

[pandaapo]

The cache named `signerToCodeSource` is never used now.

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Commit messages:
 - Make signerToCodeSource cache work.

Changes: https://git.openjdk.org/jdk/pull/11072/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11072&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296734
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11072.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11072/head:pull/11072

PR: https://git.openjdk.org/jdk/pull/11072