Spotbugs auxClassPaths eagerly resolves SourceSet compileClasspath default

[lennartfricke]

We realized that a changed compileClasspath in a sourceSet is not recognized by the spotbugs plugin.

Like when doing:
main { compileClasspath = ... }

This seems to be because the SpotbugsTaskFactory set the auxClassPaths to the value encountered on creation.
Maybe it would be better to resolve the compileClasspath lazily.

[lennartfricke]

There are some similarities to #135