Fix Caching Issues in Lombok Plugin

[tylerbertrand]

Summary

Makes the following changes to LombokConfig:

1. Updates the configFiles task input to use relative path normalization instead of absolute path normalization
2. Updates the inputPaths task input to return relative paths instead of absolute paths

Motivation

The usages of absolute paths were causing cache misses in situations that should have been cache hits. For instance, running the task on a CI machine that writes to a remote cache, and then running the same task from a local machine that reads from the remote cache. The absolute path of the two projects is different, which causes a cache miss here, even though the code is identical between them.

Tests

1. Executed experiment 3 of Gradle's build validation scripts against a test project using a snapshot version of the plugin including the changes in this PR. The test project was built twice, with no code changes between builds, but with the project relocated on disk between builds. With these changes, there are no input differences between the builds, and in the second build, the generateEffectiveLombokConfig task came from cache. Executing the same experiment, using the same test project, but with version 8.6 of the plugin, shows input differences between the two builds, and the generateEffectiveLombokConfig task was executed in the second build.