[MSHADE-434] Strange side-effect on rat-plugin

**[Chesnay Schepler](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=zentol)** opened **[MSHADE-434](https://issues.apache.org/jira/browse/MSHADE-434?redirect=false)** and commented

When upgrading the shade-plugin from 3.2.4 to 3.3.0, without changing anything else, the Apache Flink builds are getting failed by the rat-plugin, complaining about a number of files in the root target/ directory.

These files do exist, but are usually excluded by the plugin.
For some reason upgrading the shade-plugin is changing this behavior.

```java
[WARNING] Files with unapproved licenses:
/home/chesnay/dev/repos/flink/flink/target/rat.txt
/home/chesnay/dev/repos/flink/flink/target/checkstyle-result.xml
/home/chesnay/dev/repos/flink/flink/target/checkstyle-cachefile
```

This is reeeeally strange, and were wondering if you guys have any ideas for what could going on here.

Interestingly enough explicitly listing these files (e.g., `**/rat.txt`) results in them being excluded again.

This happened with Java 8 + Maven 3.2.5 and Java 11 + Maven 3.8.6 and is 100% reproducible.

---

**Affects:** 3.3.0

**Issue Links:**
- [MSHADE-321](https://issues.apache.org/jira/browse/MSHADE-321) Always respect 'createDependencyReducedPom' flag
(_**"is caused by"**_)
- [FLINK-30083](https://issues.apache.org/jira/browse/FLINK-30083) Bump maven-shade-plugin to 3.4.1

Start by reproducing the upgrade from maven-shade-plugin 3.2.4 to 3.3.0 with the Java and Maven versions listed. Inspect how the shade-plugin interacts with the rat-plugin and the exclusion of target/rat.txt, target/checkstyle-result.xml, and target/checkstyle-cachefile; compare the behavior with an explicit **/rat.txt exclusion and the change linked to MSHADE-321.

Written by the indexing model from the issue text.