Remove dependency on Google Guava

[joschi]

Context

While Google Guava provides a lot of useful functionality, it is a rather heavy-weight dependency and was notorious for breaking backward-compatibility in the past.

There are only very few features from Google Guava used in git-commit-id-maven-plugin which could either be replaced by native Java 8 features (Files.createParentDirs()) or removed altogether (@VisibleForTesting).

Contributor Checklist

• Added relevant integration or unit tests to verify the changes
• Ensured that tests pass locally: mvn clean package
• Ensured that the code meets the current checkstyle coding style definition: mvn clean verify -Pcheckstyle -Dmaven.test.skip=true -B

[TheSnoozer]

Thank you for your contribution! Looks good for me! Thanks!