Using relative paths to describe the classpath in the error message from ResolveMainClassName hinders problem diagnosis

[wilkinsona]

Looking at #39635 has led me to reconsider the contents of the error message that's produced when the main class name cannot be resolved. The message includes the classpath that was searched but each path is displayed relative to the project's root. In a complex, multi-module project this can hinder problem diagnosis as the exact directories that were searched are not immediately clear.

We can improve the situation by using absolute paths and update the tests. This will also have the side-effect of removing a getProject() call which should address #39635 but in a way that we can reproduce and test.