Clean up inconsistencies in relative file handling

**Michael Osipov** ([Bug 64841](https://bz.apache.org/bugzilla//show_bug.cgi?id=64841&redirect=false)):
Many elements like CSV Dataset, Include Controller, Script Filename and others take in a relative filename, but they handle them inconsistently. Some use the JMeter script's basedir while others use 'user.dir'. This should be consistently used troughout with the basedir of the JMeter script to support module setups.

For instance, repeating Groovy Pre/Post Processors cannot be reused because I cannot properly reference a Groovy file relative to the script.

This likey can only be resolved for 6.0.

OS: All

Review relative filename handling in the CSV Dataset, Include Controller, Script Filename, and Groovy Pre/Post Processors, starting with how each resolves paths. Compare their use of the JMeter script basedir versus user.dir. Done means these elements consistently resolve relative files from the script basedir, including reusable Groovy files for module setups.

Written by the indexing model from the issue text.