Close file lists

[pfeuffer]

Proposed changes

Though we could not reproduce an error even with real big file
structures (100 directories, containing each another 100 directories
with 100 files each), it may be safer to make sure the streams will
really be closed in the end and recommended in the javadoc description
for Files#list
(https://docs.oracle.com/javase/8/docs/api/java/nio/file/Files.html#list-java.nio.file.Path-)

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

100.0% Coverage
0.0% Duplication