We needed to mark the Clojure/ClojureScript directories as part of
allJava SourceDirectorySet on each SourceSet, for Intellij's Gradle
integration to pick them up as source roots.

This fixes #160 and #108 and #66.

Related issues:

Contributor Checklist

• Review Contributing Guidelines.
• Commits contain discrete changes, messages include context about why the change was made and reference the relevant issues.
• Commit messages should be prefixed with one of the following (these are used to determine the next version we release):
  • patch: if the change added no new functionality and is backwards compatible
  • minor: if the change added new functionality and is backwards compatible
  • major: if the change is not backwards compatible
  • chore: if the change doesn't affect plugin logic/behavior at all (and obviously still backwards compatible)
  • Any of these can optionally specify an area of the plugin they affected in parentheses (e.g. patch(clojurescript): )
    • build
    • docs
    • clojure
    • clojurescript
    • common
    • repl
• Provide functional tests. (under clojurephant-plugin/src/compatTest)
• Update documentation for user-facing changes. (under docs/)
• Ensure all verification tasks pass locally. (./gradlew check)
• Ensure CI builds pass on all Java versions. (watch the checks tab once the PR is opened)

TIP: If troubleshooting a CI failure, look for the build scan URL in the workflow run summary.