Fix and polish Log4j2PluginsCacheFileTransformer

[Goooler]

• Follow up Migrating Maven transformer to Gradle. #393 (comment).
• Closes Log4j2PluginsCacheFileTransformer not merging .dat files? #427.
• Closes Using the Log4j2PluginsCacheFileTransformer leads to multiple Log4j2Plugins.dat files #640.
• Closes Error merging log4j2plugins.dat file #735.
• Closes Log4j2PluginsCacheFileTransformer does not work anymore Goooler/shadow#104.

Updated to

• https://github.com/apache/logging-log4j-transform/blob/a190ba8dfd8a58f1ce95c3d06bcd6a924a416db5/log4j-transform-maven-shade-plugin-extensions/src/main/java/org/apache/logging/log4j/maven/plugins/shade/transformer/Log4j2PluginCacheFileTransformer.java
• https://github.com/apache/logging-log4j-transform/blob/8d6538acc50ed819d21987e69fe94b485ee0d368/log4j-transform-maven-shade-plugin-extensions/src/test/java/org/apache/logging/log4j/maven/plugins/shade/transformer/Log4j2PluginCacheFileTransformerTest.java

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• CHANGELOG's "Unreleased" section has been updated, if applicable.